stainless steel manufacturing processes

8/9/2019 Stainless Steel Manufacturing Processes

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Stainless steels

Stainless steel is a family of alloys of iron that contains at least 10.5% Chromium and a

maximum of 1.2 % carbon which is essential of ensuring formation of a self healing surface

passive layer. his passive layer provides the corrosion resistance. hese characteristics ma!estainless steels totally different from mild steels.

he stainless steel was discovered between 1"00 and 1"15. #n 1"0$ &eon 'uillet discovered

alloys with composition similar to steel grades $10 $20 $$2 $$( and $$0)C. #n 1"0( he also

discovered an iron)nic!el)chromium alloy which was similar to the *00 series of stainless steel.#n 1"0" 'iesen researched on the chromium)nic!el +austenitic *00 series, stainless steels. #n

'ermany in 1"0- onnart/ orchers found that a relationship exists between a minimum

level of chromium +10.5%, on corrosion resistance as well as the importance of low carboncontent and the role of molybdenum in increasing corrosion resistance to chlorides.

Stainless steel production process

Stainless steel is produced in an electric arc furnace where carbon electrodes contact recycled

stainless scrap and various alloys of chromium nic!el and molybdenum etc. depending on thetype of stainless steel. current is passed through the electrode and the temperature increases to

a point where the scrap and alloys melt. he li3uid steel can also be produced in &4 converter

using hot metal as a maor input material. he li3uid steel from the electric arc furnace or &4

converter is then transferred into an 64 +rgon 6xygen 4ecarboni/ation, converter where thecarbon levels are reduced and the final alloy additions are carried out to achieve the desired

chemistry. he li3uid steel is either cast into ingots or continually cast into slabs or billets. he

slabs or billets are either hot rolled or forged into the final shape. Sometimes hot rolled strips are

further processed by cold rolling to further reduce the thic!ness as in sheets and some materialsare further drawn into smaller diameters as in rods and wire. ost stainless steels are annealed

and pic!led with acid to remove furnace scale from annealing. his helps promoting the passivesurface film that naturally occurs.

Characteristics of stainless steels

Stainless steels are characteri/ed by corrosion resistance aesthetic appeal heat resistance low

life cycle cost full recyclability biological neutrality ease of fabrication cleanability and good

strength to weight ratio. 7ig 1 gives a view of various stainless steels. he family of stainlesssteels can be grouped into four types as given below. 8ach of these types has specific properties

and a basic grade.

ustenitic

artensitic

7erritic


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4uplex stainless steels.

Fig 1 Types of stainless steels

Austenitic stainless steels ) hese are iron based alloys with nic!el content ranging *.5% to

*2% chromium content ranging 1(% to 2-% and carbon content not more than 0.1%. (5% ofglobal consumption of stainless steel is of this type. he most common grades are 1-9- and

1-910. he microstructure of these types of steels is austenite. hese steels are non magnetic and

can not be hardened by heat treatment but can be hardened by cold wor!ing. hese steels havebetter corrosion resistance and can be welded. hese steels have good ductility and toughness.

hese steels are having good hygienic properties and cleanability. Stainless steels of this type are

having good resistance to low +cryogenic, and high +melting point of the alloy, temperaturessince they retain the austenitic structure. he common uses of these steels are !itchen sin!s

architectural applications such as roofs and gutters doors and windows tubular frames foodprocessing e3uipment food preparation areas chemical vessels ovens heat exchangers etc.

hese steels are designated by the following three different systems

etallurgical structure : ustenitic

'rades; Such as *0$ +most commonly used, *10 +mainly for high temperature, *1( +for

better corrosion resistance, and *1< +for better corrosion resistance,

=nified numbering system +=>S,; Such as S*0$00 S*1000 S*1(00 S*1


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iv, ?resence of very high levels of halide ions especially the chloride ions can result

into the brea!down of the passive surface film.

Martensitic stainless steels ) hese iron based alloys have chromium content ranging 11.5% to1-% and carbon content 0.15% to 1.2%. olybdenum can also be used in type of stainless steels.

hese stainless steels are magnetic in nature can be hardened by heat treatment for strength andhardness and have poor welding characteristics. he main uses for this type of stainless steels are

!nife blades surgical instruments fasteners shafts and springs etc. hese types of stainless steelsare designated by the following three systems;

etallurgical structure : artensitic

'rades; Such as $10 +most used, $20 +cutlery, $$0C +for very high hardness,

=nified numbering system +=>S,; Such as S$1000 S$2000 S$$00$

Ferritic stainless steels )#n ferritic grades of stainless steels carbon content is !ept low +less than0.0-%, and chromium content can range from 10.5% to *0%. Some ferritic grades of stainlesssteels contain molybdenum up to $%. Chromium is the main alloying element in these grades.

ecause of low carbon content these grades have a different metallurgical structure. hese steels

are magnetic in nature and cannot be hardened by heat treatment. hey are always used in theannealed or softened condition. he weldability of these steels is poor. hese steels are chosen

when toughness is not the primary need but corrosion resistance especially to chloride stress

corrosion crac!ing is important.he most common uses of these steels are automotive exhaust

and fuel lines architectural trim coo!ing utensils hot water tan!s and ban! vaults etc. hesestainless steels are designated by the following three systems;

etallurgical structure : 7erritic

'rades; Such as $0" +high temperature, $*0 +maor uses,

=nified numbering system +=>S,; Such as S$0"00 S$*000

Duplex stainless steels ) #n duplex steels carbon is !ept at a very low level +less than 0.*%, and

the chromium content is !ept high +21 to 2(%,. he nic!el content in these steels is !ept low

+*.5% to -%,. hese steels may contain molybdenum up to $.5%. hese steels have a mixedstructure which is called duplex and is a combination of both ferritic structure +50%, and

austenitic structure +50%,. hese steels have physical properties which reflect this structure.

hese steels have high resistance to stress corrosion crac!ing increased resistance to chloride ionattac! good weldability and have higher tensile and yield strengths as compared to austenitic

and ferritic stainless steels.he main uses of these steels are marine applications heat

exchangers desalination plants petrochemical plants paper plants and food pic!ling plantsetc.hese steels are designated by the following three systems;

etallurgical structure : 4uplex


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'rade; Such as 2205

=nified numbering system +=>S,; Such as S*1-0*

Austenitic Stainless Steels

ustenitic stainless steels are the most common and widely !nown types of stainless steels. hey

ma!e up over


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Fig 1 Schaeffer- Delong diagram

he family of austenitic stainless steels is shown in 7ig 2.

Fig 2 Family of austenitic stainless steels

ustenitic stainless steels are mainly segregated into the following two series

200 series : Stainless steels with a low nic!el and high nitrogen content are classified as

200 series. hese are chromium)nic!el)manganese austenitic stainless steels. 'rade 201

is hardenable through cold wor!ing while the grade 202 is a general purpose stainless

steel. 4ecreasing nic!el content and increasing manganese results in wea! corrosion

resistance.

*00 Series : he most common austenitic stainless steels are iron)chromium)nic!el steels

and are widely !nown as the *00 series. #n this series the most widely used austenitic

stainless steel is the grade *0$ also !nown as 1-9- for its composition of 1- % chromium

and - % nic!el. he second most common austenitic stainless steel in this series isthe grade *1( also called marine grade stainless steel used primarily for its increased

resistance to corrosion. typical composition of 1- % chromium and 10 % nic!el

commonly !nown as 1-910 stainless is often used in cutlery and high 3uality coo!ware.


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esides the above two series there are super austenitic stainless steel grades which exhibit great

resistance to chloride pitting and crevice corrosion because of high molybdenum content +@ ( %,

and nitrogen additions. Aigher nic!el content ensures better resistance to stress)corrosioncrac!ing than the stainless steels of the *00 series. he higher alloy content of super austenitic

steels ma!es them more expensive.

he straight grades of stainless steel contain a maximum of 0.0- % carbon. #n these grades

there is no re3uirement of minimum carbon in the specification.

he B& grades are used to provide extra corrosion resistance after welding. he letter B& after a

stainless steel grade indicates low carbon +as in *0$&,. he carbon is !ept to 0.0* % or under to

avoid carbide precipitation. Carbon in steel when heated to temperatures in what is called thecritical range +$*0 deg C to -


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ll austenitic stainless steels contain a small amount of ferrite. Conventional austenitic stainless

steel grades may contain traces of delta ferrite for improved weldability. =sually this amount of

ferrite is not enough to attract a normal magnet. Aowever if the balance of elements in the steelfavours the ferritic end of the spectrum it is possible for the amount of ferrite to be sufficient to

cause a significant magnetic response. lso some types of stainless steels are deliberately

balanced to have a significant amount of ferrite.

Properties and of stainless steels

ustenitic stainless steels are non magnetic and are not heat treatable. hey cannot be hardened

by heat treatment. Aowever they can be cold wor!ed to improve hardness strength and stress

resistance. solution anneal +heating within the range 1000 deg C to 1200 deg C followed by3uenching or rapid cooling, restores the stainless steels original condition including removal of

alloy segregation and re)establishment of ductility after cold wor!ing. Stainless steels can be

subected to solution annealing. 4ue to the solution annealing the carbides which may have

precipitated +or moved, at the grain boundaries are put bac! into solution +dispersed, into the

matrix of the metal by the annealing process. B& grades are used where annealing after weldingis impractical.

ustenitic stainless steels can be made soft enough +i.e. with yield strength of around 200 >9s3

mm, to be easily formed by the same tools that wor! with carbon steel but they can be madeincredibly strong by cold wor! up to yield strengths of over 2000 >9s3 mm. heir austenitic

+fcc face centered cubic, structure is very tough and ductile down to absolute temperature. hey

also do not lose their strength at elevated temperatures as rapidly as ferritic +bcc body centeredcubic, iron base alloys.

ustenitic grades of stainless steels are the most common used grades mainly because they

provide very predictable level of corrosion resistance with excellent mechanical properties. heleast corrosion resistant versions can withstand the normal corrosive attac! of the everydayenvironment that people experience while the most corrosion resistant grades can even

withstand boiling seawater.

ustenitic stainless steels have good formability and weldability as well as excellent toughness

particularly at low or cryogenic temperatures. ustenitic grades also have a low yield stress andrelatively high tensile strength. hey have excellent corrosion resistance and excellent high)

temperature tensile and creep strength.

ustenitic stainless steels are not very strong materials. ypically their 0.2 % proof stress is

about 250 >9s3 mm and the tensile strength between 500 and (00 >9s3 mm showing that thesesteels have substantial capacity for wor! hardening which ma!es wor!ing more difficult than in

the case of mild steel. Aowever austenitic stainless steels possess very good ductility with

elongations of about 50 % in tensile tests.

ustenitic stainless steels are also highly resistant to high temperature oxidation because of the

protective surface film but the usual grades have low strengths at elevated temperatures. hose

steels stabili/ed with i and >b grades *21 and *$


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dispersion of iC or >bC which interacts with dislocations generated during creep. 6ne of the

most commonly used alloys is 25Cr20>i with additions of titanium or niobium which possesses

good creep strength at temperatures as high as ic!el which stabili/es the austenitic structure of these steels restricts their widespread usage

since nic!el increases the costs of these stainless steels.

6ther steels can offer similar performance at lower cost and are preferred in certain applicationsfor example S *-< is used in pressure vessels but is a low)alloy carbon steel with a

chromium content of 0.5 % to " %. &ow)carbon versions for example *1(& or *0$& are used to

avoid corrosion problems caused by welding. 'rade *1(&E is preferred where

biocompatibility is re3uired +such as body implants and piercings,.

ustenitic grades of stainless steels are the most commonly used grades mainly because they

provide very predictable level of corrosion resistance with excellent mechanical properties.

=sing them wisely can save the designer of a product significant cost. hese steels are userfriendly metal alloy with life cycle cost of fully manufactured products lower than many other

materials.
http://en.wikipedia.org/wiki/Marine_grade_stainlesshttp://en.wikipedia.org/wiki/Marine_grade_stainless


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ustenitic stainless steels are those steels which are commonly used for stainless application.

Some of the applications for austenitic stainless steel include the following.

Fitchen sin!s

rchitectural applications such as roofing and cladding

#nterior decoration

Doofing and gutters

4oors and windows

Fitchenware cutlery and coo!ware

enches and food preparation areas

7ood processing e3uipment

Aeat exchangers

6vens and furnace parts

Chemical tan!s

Defects in Continuous Cast Steels

Continuous casting +CC, is the process which converts li3uid steel into a solid product mainly in

the form of slab +either thic! or thin, bloom or billets. #t is one of progressive steel ma!ing

technologies which produces a cast product of a desired cross section in indefinite length. heCC process re3uires strict observance of operating procedures technological norms and

advanced production and control techni3ues. 4espite these measures the occurrence of defects

in the CC product cannot be fully ruled out. he formation and the type of defects depends on the

status of CC machine e3uipment the cast product shape and si/e the steel grade thetechnological conditions of casting such as casting temperature and speed the mould oscillation

and cooling the 3uality and properties of the casting powder etc.

defect in a CC product can be defined as a deviation in the appearance shape dimensionmacrostructure and9or chemical properties when compared with the specifications given in the

technical standards or any other normative documents in force. 4efects are detected after casting

in the CC product through visual inspection of their surface at the cooling beds by chec!ing the

surface 3uality again by visual inspection on the inspection beds or by chec!ing the chemicalanalysis and the macrostructure of the test samples in the laboratories.


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he defects in CC products generated during the solidification and cooling process lead to loss or

diversion of prime material for further processing or sale. o prevent these losses it is necessary

to analy/e the causes of the occurrence of defects for ta!ing preventive action by adoptingpreventive metallurgical technologies and constructive solutions. lso it is necessary to segregate

and remove defective product from the prime material.

defect is not always the conse3uence of a uni3ue single cause. any times the defect is the

result of the multiple interacting causes that depend on a variable number of parameters. Similardefects may have one or more different reasons while different defects may have one or more

common causes.

4efects of the CC products are formed during the production process due to several factorswhich include material related factors casting speeds and temperatures mould oscillation

casting powder segregation coefficient of solute elements phase transformation and mechanical

and thermal stresses. echanical stresses are created due to friction ferro static pressure

bending and straightening operations and roll pressure. hermal stresses are due to non uniform

cooling in the mould and9or secondary /one. Controlling water flux impinging the surface of thestrand and minimi/ing reheating of strand can lower the thermal stresses.

aterial related factors include delta to gamma phase transformation high sulphur and low

manganese9sulphur ratio high oxygen potential of li3uid steel high super heat and presence ofinclusions in the li3uid steel. #n transformation of delta +ferrite, to gamma +austenite, volume

changes and deformations ta!e place which decrease the rate of transfer of heat from the

solidified shell to the mould. his results in a non uniform thic!ness of solidified steel shell.Steels with the carbon content of 0.0" % to 0.15 % so called peritectic steels are susceptible to

the formation of defects usually longitudinal crac!s.

Aigh casting speeds decrease the thic!ness of the chill /one which can brea! in extremeconditions. he wea!ening of the chill /one is also supported by the presence of coarse nonmetallic particles and oscillation mar!s. Aigh casting temperatures +higher super heat, increase

the surface temperatures of the strand. he formed s!in becomes overheated and then thermal

and tension stress is created which causes the formation of crac!s and defects of the s!in.

he temperature of casting of li3uid steel must be maintained above the li3uidus temperature.he difference between the casting temperature and the li3uidus temperature is called superheat.

Super heat of li3uid steel plays an important role in the defects formation and it is necessary to

control it.

echanical deformations results due to insufficient lubrication and during straightening of thestrand. hey act in the longitudinal and transversal directions. hermal stress acts in the

transverse direction when the strand is rapidly cooled and his is the cause of the formation of

the defects.

Degular oscillation of the mould prevents the molten metal from getting stuc! to the mould. t a

low oscillation fre3uency of the mould the s!in can brea! or surface crac!s and oscillation


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mar!s can be formed. 4efect formation can be avoided by an increased oscillation fre3uency of

the mould and a stabili/ed casting speed.

here are several defects which can be found on a CC product. he defects in the CC productsarising during the process of continuous casting of steel can be classified as +i, surface defects

+ii, internal defects +iii, shape defects +iv, mechanical defects and +v, deviations from theprescribed chemical composition of steel.

Surface defects can be longitudinal mid face and corner crac!s transverse mid face and cornercrac!s and deep oscillation mar!s. #nternal defects can be midway crac!s triple point crac!s

centre line crac!s diagonal crac!s centre segregation and porosity casting flux inclusion and

blow holes. Shape defects are rhombodity and longitudinal depression ovality.

Surface defects in CC products need expensive time consuming surface grinding and in severecases even downgrading or reection. he reconditioning yard is often a bottlenec! in the

process and the cost associated with removing these defects by grinding is also high.

he crac!s are openings found on the CC product surface with variable length and depth and

can sometimes extend on the entire CC product on one strand or even on the full heat. hecrac!s are not always straight. hey are sometimes interrupted and continued further in a /ig/ag

way. a!ing into account the direction on which they are formed the crac!s are called

longitudinal transverse or star crac!s.

Some of the CC defects are described below.

&ongitudinal crac!s : hey are formed in the direction of extraction of the steel. he

presence of this defect causes reection of the CC product. &ongitudinal crac!s occurs

mainly due to +i, uneven primary cooling in the mould +ii, turbulent flow of li3uid steeland a meniscus level variation in the mould +iii, non uniform or very intensive secondarycooling +iv, variance in thermal conductivity coefficient along the mould length causing

une3ual advanced wear of the mould +v, casting of li3uid steel with high superheat +vi,

high speed of casting and +vii, use of the casting powder with improper characteristics.

ransverse crac!s : hese crac!s usually appear due to the tensions on the longitudinal

direction of strand. >ormally these crac!s are ground within the permissible prescribed

limits provided they are not deep. ransverse crac!s appear due to +i, the thermal stresses

+ii, variation in the meniscus level variation +iii, presence of segregation at the bottom of

oscillation mar! and +iv, friction of the strand in the mould.

Corner crac!s : hese are crac!s present in the edge of the cast steel product. hey

appear due to high temperature variations in the li3uid steel higher aluminum content in

the steel higher sulphur level in the steel non uniform edge temperature excess frictionin the edges during casting because of non uniform distribution of casting powder and

lower superheat of the steel.


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Star crac!s : hese crac!s are very fine and caused by fragile nature of the strand at high

temperatures. hey are visible only on scale free surface. he surface is usually ground

locally to remove the defect. #ntense local cooling and presence of copper at theaustenitic grain boundary cause star crac!s. o avoid the star crac!s in the cast product it

is necessary to have +i, correct correlation between the spray flow and the casting speed

+ii, a uniform layer of melted casting powder between the strand and the mould +iii,moderate secondary cooling of the strand for avoiding increase of the thermal stress.

4epressions : hese are local deformations in the cast surface. 4epressions can be

longitudinal or transverse. &ongitudinal depressions appear li!e the shallow ditches

oriented along the length of the cast product. hey occur due to the uneven heat transfer

in the mould. hese depressions can be controlled by uniform cooling in the mould bycentering of the li3uid steel et in the mould by controlling the fluctuations of the mould

steel level use of a casting powder with suitable viscosity and melting characteristics

and by regularly monitoring the degree and uniformity of the mould wear. ransversedepressions may occur cyclically along the strand length. he peritectic steels with low

carbon and high manganese contents and the stainless steels are sensitive to this defect.he transverse depressions can be caused by the fluctuations in the mould level large3uantity of casting powder and by the turbulence of steel the sub)meniscus level. hese

depressions are controlled by controlling the mould steel level having proper mould

taper use of a casting powder with suitable viscosity and melting characteristics and

proper positioning of the input no//le and its support.

lowholes : hese are cavities in the outer surface of the cast product and are often

associated with inclusions. hey are caused by presence of gases in the steel humidity

and 3uality of the casting powder variation in the mould level presence of moisture inthe tundish refractory lining. lowholes are controlled by sufficient de)oxidation of steel

use of dry casting powder use of casting powder compatible with the grade of steelgrade temperature and casting speed control of mould level fluctuations control of

no//le immersion depth avoiding the high superheat and avoiding slag foaming aroundthe no//le.

#nterruptions in the physical continuity of the cast product : his defect occurs when

there is a pause in the casting process. #t often occurs when there is a change of heat

during se3uence mode of operation. his defect is caused by a short interruption of thecasting process and occurs when there is sudden change in casting speed caused by the

variations of steel temperature in the tundish by the variations of steel level in the mould

cogging of the no//le due to high alumina levels or by the variations of casting mode.

he corrective measures are maintenance of a constant casting speed a narrow range oftemperature variation in the tundish and steel level in the tundish within the prescribed

limits.

Slag spots defects )his defect is caused by the penetration of tundish slag in the cast

product. #t is caused by high level of slag in the tundish rise in the active oxygenpercentage in the steel lowering of steel level in the tundish resulting in slag to enter the

mould and high viscosity of casting powder.


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Some of the casting defects in a slab and a billet is shown in 7ig 1

Fig 1 Some casting defects in sla! and !illet

Cold "olling of Steels

he primary purpose of cold rolling of steels is to reduce the thic!ness of the hot rolled steel

strips +normally in the range of 1.5 mm to 5 mm, into thinner thic!nesses +usually in the range of

0.12 mm to 2.5 mm, which cannot be normally achieved during hot rolling in a hot strip mill.esides reduction in thic!ness cold rolling is done for improving the surface finish of steels for

improving the thic!ness tolerances for offering a range of Btempers for improving the physical

characteristics and for preparing the strip for surface coating.

Cold rolling ma!es the cold rolled sheets a much improved product. Cold rolled steel productsoffer good control of thic!ness shape width surface finish and other special 3uality features

that compliment the need for highly engineered end user applications. o meet the various end

user re3uirements cold rolled sheets are metallurgically designed to provide specific attributessuch as high formability deep drawability high strength high dent resistance good magnetic

properties weldability enamelability and paintability etc.

Cold rolling of hot rolled steel strips is done below the recrystalli/ation temperature normally at

room temperature. #n cold rolling process usually no heat is applied to the hot rolled strip beforerolling. Aowever frictional energy at the contact surfaces of the strip being rolled gets converted

into heat. his heat may increase temperature of the strip being rolled in rapid adiabatic process

to a level of 50 deg C to around 250 deg C.

4uring cold rolling process the reduction in thic!ness is due to the plastic deformation whichoccurs by means of dislocation movement. Steel gets hardened because of the buildup of these

dislocations. his increases strength and strain hardening upto 20 %. hese dislocations reduce

the ductility of the cold rolled steel ma!ing it useless for forming operation. o recover theductility cold rolled steels need to undergo an annealing process for the relieving of the stresses

that have buildup within the microstructure during the process of cold rolling.


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he thic!ness of the hot rolled strip is important in that the properties of the final cold rolled and

annealed product is influenced by the percent of cold reduction. his means that the thic!ness of

each hot rolled coil is carefully controlled to provide the cold rolling mill with a specificthic!ness to achieve the proper percent cold reduction. mong other things the percent of cold

reduction affects the forming behavior of the product after annealing

hrough cold rolling deep drawing 3uality extra deep drawing 3uality and extra deep drawing

plus 3uality steels are produced to meet the deep drawing re3uirements for automotiveapplications in the downstream industry .

Cold rolling of plain carbon steels involves the following four steps.

?ic!ling : #n this step the scale formed on the surface of the hot rolled steel strip is

removed since its non removal has several detrimental effects on further processing ofsteel during cold rolling. his step is described in detail in separate article under lin!

http;99ispatguru.com9pic!ling)of)scale)formed)on)hot)rolled)strip)of)carbon)steel9

Cold rolling of pic!led strip : #t is done for reduction of thic!ness of the hot rolled steel

strip

nnealing : fter cold rolling annealing of cold rolled strip is done for the relieving the

stresses that have buildup within the microstructure during the process of cold rolling.

his step is described in detail in separate article under lin!

http;99ispatguru.com9annealing)of)cold)rolled)steel9

emper rolling or s!in pass rolling of annealed strip : #t is done to give desired

mechanical properties shape and surface roughness and finishing to the cold rolled

strips.

Cold rolling of pic#led strip

Cold rolling of pic!led hot rolled strip is accomplished by processing steel strip through a rolling

mill which has an entry end reel for uncoiling the pic!led hot rolled coil and an exit end reel forcoiling of the cold rolled strip. #n between there are one or more +normally up to ( nos., rolling

mill stands for carrying out the cold reduction. 8ach mill stand has vertically stac!ed rolls that

are powered by huge motors to impart high compressive stresses into the strip. ill stands can be

2) high $)high or six)high. $)high stands are more widely used since they give maximumadvantage over other two types.

Single or two stands cold rolling mills are normally reversing mills. reversing mill is where the

steel enters the rolling mill from one side passes through to the other side and then comes bac!

through the mill again. 4uring each pass through the mill the direction of roll movement isreversed. #n this operation of the reversing mill the pic!led strip is passed forth and bac!

between mandrels on each side of the single or two stand mill. he strip is reduced in thic!ness

on each pass until the final re3uired thic!ness is attained.
http://ispatguru.com/pickling-of-scale-formed-on-hot-rolled-strip-of-carbon-steel/http://ispatguru.com/annealing-of-cold-rolled-steel/http://ispatguru.com/pickling-of-scale-formed-on-hot-rolled-strip-of-carbon-steel/http://ispatguru.com/annealing-of-cold-rolled-steel/


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ulti stand mills +normally three to six stands, are usually continuous tandem mills. Aot rolled

pic!led strip is fed into the tandem cold rolling mill from an entry end reel and progressively

reduced in thic!ness by a preset percentage in each stand to achieve the final desired thic!ness asthe strip exits the last stand. fter the last stand the strip is recoiled in the coiler.

7or rolling of high alloy and stainless steels G mill or Send/imir mill is used which operateswith a very small diameter wor! roll normally about 50 mm bac!ed up by a number of rolls +(

to 20 in numbers, in a pyramid shaped stac!. his roll set up allows extremely high forces to beexerted through the wor! roll and yet !eep the wor! roll from extreme flexing. he ta!e)up roll

on the Send/imir mill also exerts a tension on the coil as it comes through the mill. he

combination of high pressure and tension ma!es the mill capable of rolling material thin and flat.

ypical reduction of hot rolled strip in cold rolling mill can range from 50 % to "0 %. he

reduction in each stand or pass is to be distributed uniformly without falling much below the

maximum reduction for each pass. >ormally the lowest percentage reduction is ta!en in the last

pass to permit better control of flatness gauge and surface finish.

Cold rolling reduces the thic!ness of the strip by compression within the rollers. 6n the input

side the drives of the rolls need a corresponding energy supply. ecause of the high applied roll

forces the strip is heated by the forming heat to a level of up to 250 deg C. #n order to cool the

rolls and also the rolled strip they are lubricated and cooled by oil water or emulsions.8xamples for rolling oils are fat oil mineral oil or palm oilH water)free rolling oils need to have

flash points of above *00 deg C. he main reasons for lubricating are the reduction of roll forces

and roll moments the reduction of tool wear and the enhancement of the strip surface.

odern cold tandem rolling mills are capable of rolling pic!led hot rolled strips to a minimum

thic!ness of 0.12 mm at a rolling speed of up to 2500 m9min. Continuous tandem mills can have

a capacity of up to 2.5 million tons per annum.

odern cold rolling continuous tandem mills are normally e3uipped with the following features.

Aydraulic screw down system to maintain constant roll pressure and9or constant roll

position.

Computeri/ed hydraulic automatic gauge control +A'C, system. he system

automatically and consistently maintains extremely tight tolerance throughout the length

of every coil regardless of speed.

Continuous varying crown +CEC, system and enhanced shifting system

8dge drop control system

6n line strip measurement and inspection systems

4ry strip system after last stand to minimi/e the 3uantity of residual oil on the strip


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fter cold rolling the strip is highly cold wor!ed and not very useful for most applications. #t

needs to be annealed to soften the steel and ma!e it more formable.

ypical schematic of a 5 stand tandem cold rolling mill is shown in 7ig 1.

Fig 1 Typical schematics of a $ stand tandem cold rolling mill

Temper rolling or s#in pass rolling of annealed strip

Cold rolled coils after removed from the annealing furnace are in their dead soft condition andare therefore undergo a s!in pass rolling or temper rolling in a s!in pass mill. his involves acontrolled light reduction of the cold rolled steel sheet and is carried out due to the following

reason.

Strip flatness is an important property for the organi/ations which perform further

processing. his is because good flatness values allow trouble free operation of their

plant and e3uipment. S!in passing improves steel sheet flatness.

o minimi/e stretching of steel

o minimi/e straining

S!in passing causes the unsteady yield)point range !nown as the &Iders band to be

transformed into a defined yield point. his serves to improve the flow behaviour during

the deep drawing operation and to prevent unwanted lines of stress.


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o obtain desired steel surface texture. Setting the desired roughness of the strip surface

enhances the yield of the deep drawing process as well as improves the adhesion of

paints.

o obtain desired mechanical properties

o correct gauge inconsistencies in steel

emper rolling does impart a small amount of cold reduction typically in the range of 0.25 % to

1.0 %. emper rolling results in a surface that is smooth and the yield point phenomenon

+excessive stretching and wrin!ling in subse3uent operations, is eliminated. his ma!es the steel

more ductile for further forming and stretching operations. fter temper rolling the cold rolledsheet is oiled with rust preventive oil.

single s!in pass mill is independent rolling facility which usually follows the batch annealing

stage and can be implemented very flexibly. his mill can process both hard strip and ultra mild

strip. #t also provides the perfect finish for cold rolled steel strip.

special case is represented by what is !nown as the 4CD rolling mill or 4CD temper mill.

4CD stands for B4ouble cold reduction. his two stand facility combines thic!ness reduction in

the first stand with s!in passing in the second stand. #t is also possible to use both stands for s!inpassing. #n this case the steel properties such as a defined yield point are set in the first stand

while in the second stand the desired surface characteristics are transferred to the strip.

S!in pass mills can also be installed directly in line in the exit section of a continuous annealing

line. he strip can be completed in the process line and this has several advantages since theefforts and expenditure on coil handling is reduced substantially.

Cold rolled strip can be produced in various conditions such as s!in rolled 3uarter hard half

hard and full hard depending on how much cold wor! has been performed. his cold wor!ing

+hardness, is often called temper although this has nothing to do with heat treatment temper.

Juarter hard sheets can be bent +perpendicular to the direction of rolling, on itself withoutfracturing. Aalf hard sheets can be bent "0 deg while full hard can be bent $5 deg. hus these

materials can be used for in applications involving great amounts of bending and deformation

without fracturing.

%ic#el in Steels

>ic!el +>i, +atomic number 2- and atomic weight 5-.(", has density of -."02 gm9cc. elting

point of >i is 1$55 deg C and boiling point is 2"10 deg C. he phase diagram of the 7e)>i

binary system is at 7ig 1.

>i has a face centered cubic +f.c.c., crystal structure. #t is ferromagnetic up to *5* deg C its

curie point.


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Fig 1 Fe-%i phase diagram

>i is an important and widely used constituent of alloy steels. #t is best !nown as a solid solution

strengthener a mild hardenability agent and most important as a means of promoting high

toughness especially at low temperatures. >i is an important ingredient in stainless steel helping

it to prevent rust scratches and resist heat.

round (5 % of global >i production goes into the production of stainless steel.

>i alloyed steels contain as little as fraction of a percent to almost *0 % >i. s may be expected

properties of these alloy steels range from strengths similar to plain carbon steel to some of the

strongest metallic materials !nown.

6n the lower side of the >i percentage in the steels are the alloy and AS& +high strength and

low alloy, structural steels. Aot rolled steels with yield strengths of *$5 ?a may contain 0.50 %

to 2.00 % >i for toughness and added corrosion resistance. ge hardening steels contain 1.* % to

1.5 % >i plus copper +Cu, and niobium +>b,. Juenched and tempered or normali/ed and

tempered structural steels contain nic!el +>i, up to 2.25 % as well as a variety of other

constituents including chromium +Cr, molybdenum +o, or boron +,.


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>i when added to steel increases its density and hardness. #t improves steels resistance to

oxidation and corrosion. #t also improves abrasive resistance of steel.

>i is heat resistant and when combined with steel it increases the heat resistance of that steel.

>i is rust resistant ma!ing it ideal for the production of stainless steel.

ddition of >i in steel decreases the value of distortion +distortion energy, at the time of

3uenching.

Khen steel is coated with >i or plated with >i then the >i provides a scratch resistant surface to

steels.

(eat treatment

>i is a wea! hardenability agent and is rarely used alone in conventional heat treatable alloy

steels. Aowever it is often used in combination with other alloying elements such as Cr o orvanadium +E, to improve toughness. >i has profound effects on transformation characteristics

when present in higher concentrations. #t is an austenite stabili/er. #t tends to expand the gamma

loop in the iron)carbon phase diagram. t very high concentrations +greater than *0 % >i, no

ferrite is present at all.

>i has got a property of lowering the eutectoid temperature in the steel. his change point on

heating is lowered progressively with increase of >i +approximately 10 deg C for 1 % of >i, but

the lowering of the change on cooling is greater and irregular. he temperature of this change

+r1, is plotted for different >i contents for 0.2 % carbon steels in 7ig 2 and it is seen that the

curve ta!es a sudden plunge around - % >i. Steel with 12 % >i begins to transform below *00

deg C on cooling but on reheating the reverse change does not occur until about (50 deg C.

Such steels are said to exhibit pronounced lag or hysteresis and are called irreversible steels. his

characteristic is made use of in maraging steels and " % >i cryogenic steel.


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Fig 2 )ffect of %i on Ar1 and Ac1 temperatures

>i reduces the eutectoid C content therefore >i steels contain proportionately more pearlite in

its microstructure than >i free steels of the same C content.

Khile effects of >i are not strong in conventional alloy steels >i does have some influence on

heat treating transformations. #t retards both the pearlite and the bainite reactions. #t has no effect

on tempering as such but care must be ta!en when tempering >i steels at high temperatures for

extended times. =nder these conditions and because of the changes in transformation

temperatures noted above it may be possible to exceed the +lowered, c1 temperature and

inadvertently reausteniti/e the steel. he difference between the expected and actual c1 may be

as much as $5 deg C.

Aeat treatment of high nic!el alloy steels can be 3uite different from that of conventional steels.

Some examples are given below.

Cryogenic steels containing up to " % >i are used either in the 3uenched and tempered or double

normali/ed and tempered condition. #n the first case these steels are water 3uenched from -00

deg C tempered at 5(5 deg C and either air or water cooled. 4ouble normali/ing is carried out at

"00 deg C and i and $ % Co ultrahigh strength steels are hardened from -*0 deg C to -(0 deg C by

water or oil 3uenching. refrigeration treatment at )-< deg C to )(0 deg C is needed in order to

transform any residual austenite. his is a micro)structural phenomenon to which >i steels are

normally prone. 4ouble tempering at 200 deg C to (00 deg C and at 5$0 deg C to 5(5 deg C

depending on strength level needed is usually re3uired.


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hough the ultra high strength 1- % >i maraging steels are micro)structurally 3uite sophisticated

and resemble super alloys in some respects these steels are simple to heat treat. usteniti/ing

treatment is performed at around -20 deg C and is usually followed by a reasonable cooling rate.

ging for *)( hours at $-0 deg C is re3uired to complete the treatment. 4istortion is so little that

it is possible to heat)treat most of the steel parts in the finish machined condition.

Applications

>i has the ability to impart high toughness especially at low temperatures. his property has led

to the development of cryogenic steels having important applications in the transportation and

storage of li3uefied gases. >ormally the lower the service temperature that a structural steel of

this type must withstand without ris! of brittle fracture the more >i it must contain. hus a low

carbon 2.5 % >i steel can be used down to : (0 deg C while *.5 % >i lowers the allowable

temperature to )100 deg C and " % >i steels are useable up to : 1"( deg C.

mong the standard alloy steels those containing >i alone as the principal alloying constituent

are rare. #nstead >i is used in combination with other alloying elements to produce steels with

excellent combinations of strength and toughness in the 3uenched and tempered condition. ost

of these steels contain around 0.5 % >i although over * % >i is found in some grades. >i is

used in carburi/ing and nitriding steels where it benefits both case and core properties.

Steel containing 0."5 % to 1.1 % C 1 % Si 1* % to 1- % n and < % to 11 % >i which is

being a variation of Aadfield steel is being used where extreme toughness as well as high wear

resistance is important.

>i is re3uired in *00 series stainless steels to produce the austenitic structure. minimum of as -

% is sufficient but greater concentrations are re3uired when fully stabili/ed austenite is needed.

'rade *10 contains around 20 % >i while the sulfuric acid resistant steel +lloy 20, contains 2"

% >i 20 % Cr and lesser amounts of n Si o and Cu. he precipitation hardening stainless

steels are essentially unbalanced austenitic grades +similar to grade *01, containing such

hardeners as Cu o i l >b a or E. >i is also used in martensitic stainless steel grades

+grades $1$ and $*1, where the presence of up to 2.5 % >i prevents the formation of L : ferrite.

>i is rarely used in tool steels as its presence promotes graphiti/ation in these high carbon alloys

+inhibits through hardening,. Aowever some grades contain minor amounts of >i for toughnessgrain refinement and ease of heat treatment.

Calcium in Steels


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Calcium +Ca, +atomic number 20 and atomic weight $0.0-, has density of 1.5$ gm9cc. elting

point of Ca is -$2 deg C and boiling point is 1$-$ deg C.

Ca additions are made during steel ma!ing for refining deoxidation desulphuri/ation and

control of shape si/e and distribution of oxide and sulphide inclusions . Ca is not used as

alloying element since its solubility in steel is very low. 7urther it has a high vapour pressure

since it boiling point is lower than the temperature of the li3uid steel. #t has a high reactivity and

hence special techni3ues are necessary for its introduction and retention of even a few parts per

million in the li3uid steel.

dvantages directly attributable to Ca treatment include greater fluidity simplified continuous

casting and improved cleanliness +including reduction in no//le bloc!age, machinability

ductility and impact strength in the final product.

A*aila!le forms

Ca is added to steel in the stabili/ed forms of calcium silicon +CaSi, calcium manganese silicon

+CanSi, calcium silicon barium +CaSia, and calcium silicon barium aluminum +CaSial,

alloys or as calcium carbide +CaC2,. 8lemental Ca is difficult and dangerous to add to li3uid

steel.

CaSi in steel sheath +also called cored wire, is the most commonly used addition agent for Ca

addition. he cored wire is inected into the li3uid steel with help of wire inection system. #t has

higher recovery of Ca in steel than the virgin Ca 9 CaSi lumps addition into the ladle. he CaSi

cored wire contains $.5 % of iron +7e, and 55 % to (5 % of Si. Ca content is usually in three

ranges of 2- % to *1 % *0 % to ** % and *2 % to *$ %. #t contains around 1 % carbon +C, and

about 1.5 % aluminum +l,. #t has a melting point in the range of "-0 deg C to 12(0 deg C and

boiling point of around 1500 deg C. Aowever melting point range is not an overriding factor as

Ca vapouri/es so 3uic!ly that it can provide a beneficial agitation as it bubbles through the li3uid

steel.

Addition practice

he treatment with CaSi is normally made after trim additions and argon +r, rinsing in most of

the steel grades. #t is not uncommon practice to perform a preliminary deoxidation with Si and9orl before adding Ca.

#n the ladle the low density and high reactivity of Ca addition agents ma!e their efficient use

difficult and this has led to the development of special addition techni3ues. 6ne techni3ue is to

inect Ca deep into the li3uid steel bath such that the ferrostatic pressure overcomes vapour


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pressure of Ca. #n another method wire containing Ca components are inected at speeds of -0)

*00 m9min.

=nless the ferrostatic head is 3uite high +as at the bottom of the ladle, Ca exists only in vapour

form at the steelma!ing temperatures. his plus the fact that solubility of Ca in steel being low

at any temperature means that any reaction between Ca and the oxygen +6, and sulfur+S, it is

intended to remove can only ta!e place at the Ca vapour9li3uid steel interface unless Ca is

present as a component of the slag. he addition of Ca through the cored wire inection ma!e the

contact between Ca and li3uid steel as intimate as possible for as long as practical limits allow.

#t is important that all efforts be made to prevent reoxidation of the steel after the Ca addition.

=se of protective slag blan!ets inert gas or refractory shrouded no//les and even submerged

no//les are necessary if Ca protection is to be maintained. here are two reasons for this

precaution. he first reason is the reoxidation of the steel can cause S reection from calcium

sulphide +CaS, since Ca6 is more stable than CaS and proper conditions can even lead to the

reformation of manganese sulphide +nS,. he second reason is that there are a number of

thermodynamically possible calcium aluminates and excessive reoxidation favours the formation

of high melting point calcium aluminates which are ust as harmful especially in terms of no//le

bloc!age than the alumina+l26*,9silica+Si62, precipitates which the Ca was intended to

remove or modify.

'nfluence of calcium on steels

4uring calcium treatment the l26* and Si62 inclusions are converted to molten calcium

aluminates and silicate which are globular in shape because of the surface tension effect. hechange in inclusion composition and shape is !nown as the inclusion morphology control.

ddition of Ca to li3uid steel containing 6 and S forms two phases namely oxide and sulphide.

6xide phase consists of the compounds in Ca6Ll26* system +7ig 1,. he different oxide

compounds +* Ca6. l2 6* 12 Ca6.< l2 6* Ca6. l2 6* and Ca6. 2 l2 6*, have different

melting temperature which ranges from 1$00 deg C to 1


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Fig 1 Ca+-Al2+, !inary system

12 Ca6.


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titanium +i, modified grade *21 austenitic stainless steels used in welded or sensiti/ation prone

structures.

Ca is used to improve the fluidity and cleanliness of cast irons and cast steels. he violent

agitation that accompanies Ca addition to li3uid metal also reduces the gas content of the metal.

his leads to sounder and less porous cast structures.

stainless steel manufacturing processes

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