RKP-NIT-MM 325-E

Institute of Minerals & Materials Technology

Bhubaneswar, INDIA

MODERN MODERN STEELMAKINGSTEELMAKINGEOF versus BOF

SteelmakingTraditional integrated steel plants adopt BF-BOF route. However, DRI/HBI-EAF route, or, SR/EAF routes are challenging at the moment. Share of EAF route in the past:

Year: 1910 1950 1980 1995 2010 2020(expected)%: 0.2 8 20 33 34 50

Continuous efforts are being made to improve upon EAF:

A: to improve the specific energy consumption per ton of steel. B: to reduce tap-to-tap time to improve productivity (~35 min).C: for foamy slag practice, O blowing from top and side, adoption of jet-box, coherent-jet and other modifications,D: to have continuous on-line exhaust gas analysis etc.,

The result is EAF based steel plants to produce DD, EDD, SDD, special wire rod, special bar, IF, electrical wire grade steels.


Bhubaneswar, INDIA

MODERN MODERN STEELMAKINGSTEELMAKING EOF vs BOF – characteristics of

DRI

Fe% in ore vs Fe% in DRI at 92% metallization


Bhubaneswar, INDIA

MODERN MODERN STEELMAKINGSTEELMAKING

C and O present in DRI react with each other and maintain a reducing condition which is a problem in EAF running under oxidizing conditions. However, maintaining C and O in DRI in a suitable proportion, one can melt and refine DRI in EAF side-by-side as a continuous process.

The following conditions should help:

A: P in DRI should be below 0.05% and S below 0.03%.B: The DRI can substitute scrap up to 15-20% in the total charge. However, 100% DRI can also be charged in a high powered arc furnaces.C: The volume of slag depends on amount of gangue present.D: In case of continuous charging, rate of charging and rate of power/fuel supply should be synchronized.E: Charging of pre-heated DRI (~6500) is possible.

EOF vs BOF – characteristics of DRI


Bhubaneswar, INDIA

MODERN MODERN STEELMAKINGSTEELMAKINGAdvantages of DRI + HM as

charge

X axis is % hot metal in the charge


Bhubaneswar, INDIA

MODERN MODERN STEELMAKINGSTEELMAKINGAdvantages of DRI + Scrap + HM as

charge

Variation of HM proportion on productivity


Bhubaneswar, INDIA


Energy requirements in EAF


Bhubaneswar, INDIA


A: The ‘primary steelmaking processes’ such as OH, LD, OBM, LDAC, EAF etc. were once quite satisfactory and meeting the necessary requirements.

B: With time, the quality requirements continuously increased and further developments were necessary, which were met through ‘secondary steelmaking’ or ‘secondary steel refining’.

C: Some steel specifications with respect to time:

Contents/Year: 1960 1980 2000 Future (expected)

C 250 150 20 10P 300 150 50-100 30S 300 30 10 10N 150 70 30 20O(total) 30 30 10 10H 6 6 1 1

SECONDARY STEELMAKING PROCESSES


Bhubaneswar, INDIA

SECONDARY SECONDARY STEELMAKINGSTEELMAKINGA: In principle the further developments mean a duplex

practice, where,

(i) the lengthy process of refining, alloying, deoxidation etc. including melting, are handled in the ‘primary steelmaking’ , and,

(ii) the slower and finer processes such as deep de-sulphurization etc. are shifted to ‘secondary steelmaking’.

B: The history of secondary steelmaking went through ‘three distinct stages’ of development:

(a)The first stage was ‘ladle metallurgy’ and aimed at:(i) improving deoxidation control,(ii) removing inclusions by simple stirring,(iii) desulphurizing by synthetic slag and injection metallurgy,(iv) modifying inclusions primarily by calcium.


Bhubaneswar, INDIA

SECONDARY SECONDARY STEELMAKINGSTEELMAKING

Three ladles: - Ladles treatment section


Bhubaneswar, INDIA

SECONDARY SECONDARY STEELMAKINGSTEELMAKING(b) The second stage was ‘ladle furnace’ which achieved:

(i) reheating of steel and control of teeming temperature,(ii) buffer heats for sequencial casting,(iii) homogenization of heat w.r.t. chemistry and temperature,(iv) large alloying to produce high alloy steels,(v) producing ultra-clean steels with an extended and gentle gas stirring,(vi) desulphurizing, and even dephosphorizing the steel with synthetic slag with, even two slags, one after the other.

C: The third stage is ‘vacuum degassing’, where, full scale vacuum treatment is effected to produce steel for wide range applications.

D: Thus the steelmaking routes are:(i) ORE – BF – LF – (V.T.) – Con.Cast. – Roll – Product, or,(ii) ORE – BF/DRI – EAF – LF - (V.T.) – Con.Cast. – Roll – Product, or,(iii) ORE – DRI – IF – Con.Cast/Ingot – Roll – Product.


Bhubaneswar, INDIA

SECONDARY SECONDARY STEELMAKINGSTEELMAKINGA: Now-a-days secondary steelmaking is employed to achieve

one or more of the following:(a) improvement in physical quality (surface or homogeneity),(b) more close and homogeneous chemistry,(c) lower levels of impurities/tramp elements,(d) improvement in overall production rate,(e) decrease in energy consumption if any,(f) use of cheaper grade/alternative raw materials,(g) use of alternative source of energy,(h) higher recovery of alloying elements,(i) effective temperature control,(j) deeper de(Carbu/Sulphu/Phospho)rization,(k) degassing/deoxidisation/micro-alloying,(l) modification of morphology/chemistry of inclusions,(m) improvement in cleanliness,(n) control of solidification structure.


Bhubaneswar, INDIA

A: The concept of secondary steelmaking has resulted in a very fundamental change in technology of steelmaking. It has also become universal in application. They tailor to each specific requirement of product range.

B: Lately the secondary steelmaking itself is further sub-divided into two broad divisions: (a) ‘gross secondary refining’, and (b) ‘fine secondary refining’.

C: Secondary steelmaking process varieties are:(i) stirring treatments,(ii) synthetic slag treatment with stirring,(iii) vacuum treatments,(iv) decarburization techniques,(v) injection metallurgy,(vi) plunging techniques,(vii) post-solidification treatments,(ix) tundish metallurgy.



Bhubaneswar, INDIA


1: Homogenisation of temperature and chemistry

2: Minor alloying for close control of chemistry

3: Cleanliness improvement

Stirring techniques


Bhubaneswar, INDIA


A: Homogenization of bath temperature and composition is achieved because of the buoyant energy of the injected gas which is calculated as follows:

E = 14.23 [VT/M] * log[(1 + H)/1.48 P0]

Where, E = stirring power, V = gas flow rate, Nm3/m,T = bath temperature in K, M = bath weight, ton,H = depth of gas injection, m,P = gas pressure at the bath surface, atm.

The mixing time as defined by 95 % homogenization is given by:

ln sec = 116 * E-1/3 * D5/3 * H-1 (where, D is ladle diameter, m.)

The temperature drop of bath:(Mass of bath) X (Sp. Heat) X (Temp. drop) =

(Mass of gas) X (Sp. Heat) X (Bath temp.)

Stirring Tecniques


Bhubaneswar, INDIA

SECONDARY SECONDARY STEELMAKINGSTEELMAKINGStirring

Tecniques

Schematic representation of a gas-stirred ladle


Bhubaneswar, INDIA


The mechanism of inclusion removal is as follows:

A: an open eye is formed when a critical gas flow rate is achieved,

B: a layer of spheres of slag droplets covered with thin metal film is formed around the open eye (called ‘sphere bed’),

C: small inclusions are brought up to this eye by the bubbles,

D: the sphere bed acts as a filter which captures some of the inclusions - possible only if energy considerations are favorable,

E: some inclusions may take more than one lift before they are caught by the slag,

F: the system is far from still bath conditions when Stoke’s law holds good,

G: inclusions do collide, coagulate and coalesce and thereby increase in size which helps them float more easily.

Cleanliness improvement


Bhubaneswar, INDIA


A: Making stainless steel using stainless scrap has problems:

(i) utilization factor of power is low, (ii) costly low-C ferro-alloy has to be used for final adjustment, (iii) O lacing increases temperature affecting lining life, (iv) heterogeneity in the composition of bath, (v) recovery of Cr is low.

B: The reaction is:

3CO + 2[Cr] = (Cr2O3 )+ 3[C]

C: [Cr] will depend on partial pressure of CO and [C]. Further, temperature also has effect on this reaction. Any excess [Cr] will be oxidized and lost in slag.

D: When a stainless steel bath is decarburized with pure O, the atmosphere is nearly pure CO, i.e., CO at one atmospheric pressure.

Decarburization techniques


Bhubaneswar, INDIA

SECONDARY SECONDARY STEELMAKINGSTEELMAKINGDecarburization

techniques

[C] and [Cr] as a function of temperature and presuure

A: 1700 1

B: 1800 1

C: 1900 1

D: 1700 0.5

E: 1700 0.3

F: 1700 0.1

G: 1800 0.6

T, 0C, P, atm


Bhubaneswar, INDIA


The following processes are used:

A: AOD process : (i) molten metal from EAF charged to this vessel, (ii) Ar-O blown, (iii) coolants charged, (iv) Some Ar may be replaced by N, (v) Fe-Si deoxidiser, (vi) duration-2 h, (vii) 80 heats, (viii) more than 97 % Cr recovery.

B: VOD process: (i) consists of vacuum tank, ladle furnace, Ar/no Ar stirring, a lid with O lance, (ii) similar to AOD, (iii) duration – 2 to 2.5 h, (iii) more than 97 % Cr recovery. C: CLU process: (i) EAF followed by Bessemer type bottom blown furnace, (ii) Ar may be partly replaced by steam.

D: MRP process: similar to AOD, but more efficient.

E: SR-KCB process: Chrome ore is used and P is removed beforehand. Final adjustment is done in VOD.

Stainless steelmaking technologies


Bhubaneswar, INDIA


A: We know that basic Bessemer steel containing 0.018% N is not suitable for many operations, because N in steel causes strain brittleness. It is an interstitial element and strains the iron lattice and creates serious problems during rolling.

B: On the other hand, we also know that an important feature of stainless steel is retention of austenitic structure at room temperature, which is ensured by having 9% Ni in 300-series stainless steel.

C: Now-a-days, it is understood that this effect can also be obtained by having N in steel in place of costly Ni. This is called 200-series stainless steel. Examples of such steel are: Grade/UNS No. C Mn Si Cr Ni P(S) NAISI201/S20100 0.15 5.5-7.5 1 16-18 3.5-5.5 0.06(3) 0.25 %AISI/S20500 0.12-0.2514-15.5 1 16.5-18 1-1.75 -do- 0.32-0.4%

Nitrogen problem


Bhubaneswar, INDIA


D: Thermodynamics indicates that N2 dissociates on dissolution steel as: N2 = 2N so that KN = hN

2/pN2

If the dissolution obeys Henry’s Law then it can be:wt.% N = (KN x pN2)1/2 /fN

, where, fN is activity coefficient of Nitrogen.And Log KN = -(188.1/T) – 1.246.

These relationships show that N can not go above 0.045 %.How to get 0.25 – 0.4% N !!

E: It is possible to get it in presence of other elements like Mn, Cr etc. In India we get 0.25% N with 14-16% Cr, 5-6% Mn, 0.15 % C and less than 1% Si.F: There are empirical formulae to suggest the extent of substitution. One example:

%Nieq = %Ni + 0.31(%Mn) + 22(%C) + 14.2(%N) + %Cu

This is made by AOD process which is called NOD process.

Nitrogen problem(cont)


Bhubaneswar, INDIA

SECONDARY SECONDARY STEELMAKINGSTEELMAKINGInjection

metallurgy

TN (Thyssen Niederrhein) system of injection

Injection metallurgy is

applied in case of S removal and

inclusion modification

The injection lance is made of

ceramicmaterial and argon is career

gas.

For S removal Ca-Si /CaO-CaF2/CaO-

Al2O3-CaF2 are added.


Bhubaneswar, INDIA


A: Plunging is inserting a small crucible containing the reagent into the ladle. The crucible is held by a refractory rod and plunged upside down. It is practiced when the reagents are small, usually in case of S removal or micro-alloying.

Plunging Technique

Post solidification treatmentsA: Some times quality steel is produced by post-solidification

treatments, i.e., after primary refining and ingot casting. The steel is re-melted and re-cast into ingot. Such processes are:(i)Zone refining – usually adopted to produce purer metals.(ii)Vacuum arc remelting (VAR) Process – adopted for alloy steel of better cleanliness and low S. The primary ingot forms the electrode to be drip-melted into a water-cooled Cu mould. In VAR process, The arc is struck between the electrode and the mould under vacuum and refining is made and re-solidification is madethere itself. Here, the H and O contents are also very low.


Bhubaneswar, INDIA

SECONDARY SECONDARY STEELMAKINGSTEELMAKINGPost solidification

treatments(cont)(iii) Electro Slag Refining (ESR) Process – likeVAR, also

adopted for alloy steel of better cleanliness and low S. The primary ingot forms the electrode to be drip-melted into a water-cooled Cu mould. In ESR process, a slag layer is used to act as a resister between the electrode and the mould and which is responsible for melting the electrode. The slag also acts as a refining agent.

In both these processes the electrode melts progressively and is resolidified on the mould, nearly unidirectionally. Because of the high temperature, small pool of molten metal and almost unidirectional solidification, both of these processes can produce sound ingots of high density. The composition remains same, but cleanliness is improved, segregation decreased with practically no cavity.


Bhubaneswar, INDIA


Ladle Furnace (LF):

Then it is capable to carry out:

(a)stirring,(b)Vacuum treatment,

(c) Synthetic slag refining,(d) plunging, (e) Injection etc.

With Provisions like:(i) Bottom plug for Ar

purging,(ii) Lid with electrodes (arc

furnace for heating),(iii)Another lid for vacuum,(iv)Chutes for additions,(v) Opening for injection


Bhubaneswar, INDIA


ASEA-SKF LF FURNACE

Essentially a teemingLadle Provided with:

(i) Electromagnetic stirrer(therefore, the ladle shellis made up of austenitic stainless steel),

(ii) Lid with electrodes (arc furnace for heating),

(iii)Another lid for vacuum,(iv) Steam ejector unit

(for evacuation).


Bhubaneswar, INDIA


Sequential operation of an SKF:

The feed is nearly refined steel

(a) tapping primary furnace into the SKF ladle directly,(b) controlled stirring during the entire secondary processing,(c) vacuum treatment including minor decarburization,(d) extensive decarburization for stainless steelmaking,(e) deoxidation,(f) desulphurization and deslagging,(g) alloying to desired extent,(h) temperature adjustment,(i) teeming from the same SKF ladle.

ASEA-SKF LF FURNACE – OPERATION:


Bhubaneswar, INDIA

CASTING PIT PRACTICECASTING PIT PRACTICEOnce refining is over, the molten steel is to be

cast.

The conventional casting process involves: (a) casting in a metallic or sand mould in a foundry – consumption is small, (b) casting into a metallic mould and then rolling/forging. The hot followed by cold rolling produced slabs and sheets for further manufacturing.

The entire process starting from tapping till solidification is called ‘casting pit practice’ or ‘pit-side practice’.

The efficient and high yielding continuous casting of steel into billets, blooms, slabs, thin slabs, sheets, and now even thin strips (2 mm thick) have taken over the conventional casting process in integrated as well as mini steel plants.

Even though the old type operations are almost extinct, the entire operation is still called in the same name.


Bhubaneswar, INDIA

CASTING PIT PRACTICECASTING PIT PRACTICE

Teeming Ladle


Bhubaneswar, INDIA


The Stopper Assembly

for teeming ladles:

The details of the stopper

assembly and the stopper

end are shown:


Bhubaneswar, INDIA


Ingot mould types

(b) WIDE END UP & NARROW END DOWN


Bhubaneswar, INDIA


Hot tops of various designs


Bhubaneswar, INDIA


Constructional details of bottom teeming of heights

VerticalSection

(a)

BottomPlates-

twoDifferentDesigns(b)


Bhubaneswar, INDIA


Structure of a killed ingot with three zones of solidification


Bhubaneswar, INDIA

The three zone structure is altered when O is there

The reaction: [C] + [O] = [CO]

The gas evolved changes the structure and the following 8 types of ingot are produced:

(a) Type-1: killed steel-cast in wide-end-up moulds,(b) Type-2: semi-killed – small gas evolves at the end,(c) Type-3: little more than 2, burst the top solidified surface,(d) Type-4: gas appreciable, metal cap is placed at the top, series of honeycomb shaped blow holes along the walls,(e) Type-5: large gas, capped, thick skin, then honeycomb,(f) Type-6: capped, gas escapes, thick skin at lower portion,(g) Type-7: brisk gas evolution, very thick skin and honeycomb,(h) Type-8: rimmed steel, very violent rimming action, no blow hole, ingot shrinks.

Ingot types based on deoxidation level:



Bhubaneswar, INDIA


Typical ingot structure


Bhubaneswar, INDIA

The final structure of ingot depends on degree ofde-oxidation carried out prior to solidification.

The control of structure therefore is regulated as follows:

(a) Rimming steels: It should have a lot of gas evolution during solidification. So C in steel must be below 0.15 %. Regulated by maintaining proper amount of iron oxide in slag.

(b) Capped steels: It is another variety of rimming steel. Gas evolution is less brisk. Casting is done in a bottle shaped narrow-end-up mould. Cast iron cap is provided. C is ~ 0.15 %.

(c) Semi-killed steel: Small amount of gas is evolved. C in the range of 0.15 – 0.30 %.

(d) Killed steel: No gas evolution. C should more than 0.3%.

Control of ingot structure:



Bhubaneswar, INDIA

It is aimed to produce both physically and chemicallyhomogeneous ingot which would have (i) required structure, (ii) free of chemical segregation, (iii) free of metallic inclusions,cavities etc. and would have smooth surface finish.

However, in practice defects in ingots come up:

(a) Pipe: Volumetric contraction appearing as cavity. These are reduced by (i) adopting a top hot feeder head (pipe gets confined there), (ii) use of insulating or exothermic materials at the top, (iii) may add extra metal after partial solidification.

(b) Blow holes: They occur due to gas evolution – two types(1) primary (honeycomb near skin), (2) secondary (more spherical and located deeper inside). Can be welded during rolling.

(c) Columnar structure or ingotism: Dendritic growth of crystals developing to columnar structure.

Ingot defects and remedies:



Bhubaneswar, INDIA

(d) Segregation: Departure from the average composition. May be +ve and -ve. Chill layer has no segregation. Progressive solidification results in segregation. There may be (1) zone of +ve segregation, (2) zone of -ve segregation, (3) ‘V’-type or (4) ‘A’-type segregation. Can be minimized by prolonged soaking.(e) Non-metallic inclusions: Can be indigenous (arising in the steelmaking), exogenous (mechanical erosion of refractories) – oxides, sulphides, nitrides etc. Proper de-oxidation and use of proper refractory minimises these inclusions.(f) Internal fissures and hair-line cracking: These are called ‘clinked ingot’ and formed due to rapid heating or rapid cooling.(g) Surface defects: (1) ingot cracks: (i) longitudinal cracks, (ii) transverse cracks, (iii) restriction cracks, (iv) sub-cutaneous cracks, (2) other surface defects: (i) scab, (ii) lapiness, (iii) splash, (iv) crazing, (v) double skin, (vi) spongy top, (vii) falsh, (viii) boot leg, (ix) skin holes.

Ingot defects and remedies (cont):



Bhubaneswar, INDIA

Gases, O, N, H, CO enter/generate in steel at various stages.

Those can be removed by vacuum treatment of liquid steel.

Principle: % gas dissolved = (partial pressure of gas at ambient atmosphere)1/2.

Degassing Processes:(a) Ladle degassing: Conducted in a ladle put under a cacuum chamber. Can be stirred by bubbling an inert gas or by electro-magnetic stirrer.(b) Stream degassing: The liquid steel flows like a stream from the furnace or ladle to another ladle or mould during vacuum.(c) Circulation degassing: Liquid steel is continuously or intermittently circulated during its exposure to vacuum. The circulation may be brought about in several ways like in D-H/R-H continuous degassing circulation systems.

Gases in steel and vacuum degassing:



Bhubaneswar, INDIA

Conventional ingot casting has the following disadvantages:

(1)Large capital cost due to moulds, bottom plates, rails, cranes etc.

(2) In spite of all care defects occur in ingots.

(3) Certain portions (ends of ingots) has to be discarded (low yield).

(4) If billets are needed additional rolling facilities needed.

The answer was ‘Continuous Casting of Steel’.

The Principle: Continuous casting is teeming of liquid metal in a short mould with a false mould with a false bottom through which partially solidified ingot is continuously withdrawn at the same rate at which the metal is poured in the mould.

CONTINUOUS CASTINGCONTINUOUS CASTING


Bhubaneswar, INDIA

The equipments for continuous casting of steel consists of:

(1)The ladle to hold steel for teeming,

(2) The tundish to closely regulate the flow of steel to the mould,

(3) The mould to allow adequate solidification of the product,

(4) The withdrawal rolls to pull out the ingot continuously from the mould,

(5) The cooling sprays to solidify the ingot completely,

(6) The bending and/or cutting devices to obtain handlable lengths of the products,

(7) The auxiliary electrical and/or mechanical gears to help run the machine smoothly.



Bhubaneswar, INDIA


Solidification characteristics of continuously cast ingot


Bhubaneswar, INDIA

The casting speed is important. The casting speed can be increased if a moving mould is used. The principle of moving themould is known as Junghan’s Principle. In this the mould ismoved up and down variously through a stroke of 3-10 mm. TheDownward speed is more than the speed of withdrawal whichleads to ‘negative stripping’ of the ingot from the mould. The negative stripping is advantageous in the following ways:

(1) The initially crystallized skin of the ingot is further compacted.

(2) Formation of tensile stresses is prevented (even compressive may be developed) in the initially solidified skin.

(3) It practically eliminates the possibility of transverse cracking of the ingot skin.

(4) Transverse cracks that may be formed earlier are liable to be welded again.

(5) It allows maximum rate of withdrawal (production) from a given machine.



Bhubaneswar, INDIA

Continuous casting operation is adjusted with respect to the following variables:

(1)Chemistry of the steel being cast.

(2) Temperature of steel at teeming (all through).

(3) Number of strands cast in parallel and simultaneously.

(4) Mould size and dimensions. (5) Mould stroke.

(6) Casting speed (rate of withdrawal = productivity).

(7) Frequency of mould stroke (per min).

(8) Negative strip time. (9) Positive strip time. (10) Total cycle time.

(11) Negative strip ratio, (12) Casting powder viscosity.

(13) Powder consumption.

(14) Thermal conductivity of the steel being cast.



Bhubaneswar, INDIA


Vertical type continuous casting

machine

Types of continuousCasting machines:(i) Vertical-type, (ii)Vertical mould and

horizontal-discharge-type,

(iii) Curved mould (S-type)


Bhubaneswar, INDIA

CONTINUOUCONTINUOUS S

CASTINGCASTING

Vertical-mould-bend-discharge Type continuous casting machine


Bhubaneswar, INDIA

The Curved Mould (S-type):This is the latest design now almost universally adopted for

continuous casting of almost any section like billets, blooms and slabs. The characteristics of this machine are:

(1)The mould in itself is curved mould rather than straight one employed in the earlier two designs.

(2) The strands come out of the mould in curvilinear fashion with a fixed radius.

(3) It is bent before the entire cross-section is solidified.

(4) The curved strand is in fact straightened after it is fully solidified and cooled to the designed extent.

The height of the shop in this case is minimum as compared to that of the other two designs and hence is called ‘low head’ type machine. However, ‘S-type’ machine is the popular name.



Bhubaneswar, INDIA


The Curved Mould (S-type) machine (at TATA STEEL)


Bhubaneswar, INDIA

Essential details of continuous casting machine:

The basic requirements of any continuous casting machine are as follows:

(1)Hot metal handling system as a source of molten finished steel.

(2) Tundish for supply and distribution of liquid steel to the mould.

(3) Mould to freeze the skin of the casting.

(4) Water sprays to complete solidification and required cooling.

(5) Drive system to withdraw the strand continuously at a pre-determined rate.

(6) Cut off machine to cut the continuously solidified piece into required length.



Bhubaneswar, INDIA

Molten steel handling system:

(1): Earlier days the lip poured or the stoppered teeming ladles were used for supplying liquid steel to the machine, but now, bottom poured teeming ladles are universally used. Since use of LF for secondary refining is a common practice, this ladle is used straightaway.

(2): The lining ,earlier, was firebricks, but now, dolomite lined ladles are used to control dissolved O. Results of TATA STEEL shows 2-6 ppm O in dolomite/bauxite lined ladles, which is 10-20 ppm in case of silica lined ladles.

(3): The LF is usually lined with carbonaceous magnesia bricks. If DD steels with extra low C is the product then VOD is adopted for the last leg of de-carburization and hence. In that case dolomite lining should haveto be used for the ladle while processing under vacuum.



Bhubaneswar, INDIA

Tundish:

(1): Tundish is an intermediate container after the teeming ladle, but before the mould. It helps: (i) in controlling the flow properly by maintaining a constant level of HM during the entire teeming period, (ii) in sub-dividing the flow into several streams so that simultaneous casting can be made in all these strands, (iii) in teeming a single large mould by several streams from the same tundish, (iv) in deflecting metal streams to a receptacle in case of machine breakdown.

(2): The tundish is pre-heated before use to minimize heat losses during initial period of teeming so that any freezing of steel in the nozzle area is prevented. Tundish covers are also provided.

(3): Even if a slide gate of a tundish fails spare tundish can be used to complete the teeming. (4): The metal level in a tundish can be altered readily to effect adjustments in rate of teeming.



Bhubaneswar, INDIA

Tundish (cont):

(5): A modern day tundish also effects cleaning of the steel, also helps in removing the slag, if any has come from the ladle. For removing inclusions the residence time of steel in tundish is increased by: (a) improving flow pattern, (b) using suitable fluxes to absorb inclusions, (c) using ceramic filters to trap inclusions, (d) minimizing vortexing.

(6): Use of tundish allows final deoxidation adjustments, trimming additions, desuphurization, inclusion modification and temperature control.

(7): There are various designs of tundish like, single strand, multiple strand, T-shape, Delta-type, V-shape etc.

(8): There are two methods of tundish lining such as (i) conventional lining which requires pre-heating and (ii) the cold castable lining which does not need pre-heating, but needs insulating powder cover on the steel bath. A well designed, lined and maintained tundish has a life of 40-50 heats.



Bhubaneswar, INDIA


A modern tundish showing location and use


Bhubaneswar, INDIA


Mould of continuous casting machine

Made out of drawn Cu tube or machined out of a solid block or welded plate construction of high conductivity electrolytic grade Cu.

Open bottom – closed by a dummy plug in the beginning.

Nearly 70-140 mm in length.

To avoid transverse cracking it is oscillated at the rate of 30-60 oscillations per minute with a negative strip.

Modern moulds are tappered to narrow down towards the bottom to accommodate shrinkage.


Bhubaneswar, INDIA

Mould: The primary functions of a mould are to contain and start the steel solidification, during the teeming period, with the following aim:

(1): To obtain enough shell thickness of solidified steel to stand bending stresses and ferro-static pressure of liquid steel from inside.

(2): To equalise temperature all through liquid steel mass.(3): To ensure internal and surface quality of the product.

Moulds are invariably lubricated by moisture-free rape-seed oil.

Mould specifications for billet caster at TATA STEEL:Mould size: 801 mm long curvedTaper: 0.4%/m for 100 mm sq and 0.6%/m for 125 mm

sqCasting radius: 6 m Metallurgical length: 16.2 m



Bhubaneswar, INDIA

Submerged Entry Nozzle (SEN): (1): In the beginning casting was made in open air which gave

rise to defects due to oxidation of metal stream in open atmosphere. This was avoided by covering the teeming area and passing a neutral or reducing gas.

(2): Subsequent development was ‘Submerged entry nozzle (SEN)’. It is common use now. It is a ceramic (Al2O3-C, or, ZrO2-C) tube having a closed bottom, connected to the tundish at one end, while the other end is dipped in the molten pool in the con-cast mould. It has openings on side-ways, near the bottom, for the lateral flow of liquid steel into the mould.

(3): In slab casting SEN has two openings in opposite direction towards the smaller opposite walls.

(4): In case of blooms it may have four openings to distribute the steel uniformly in the entire cross-section of the mould.



Bhubaneswar, INDIA


Submerged Entry Nozzle (SEN)


Bhubaneswar, INDIA

Mould lubrication: In earlier days moisture-free rape-seed oil was used as lubricant. Now-a-days, fluxes have been developed and used for smooth casting operation. These are added on the molten steel surface in the mould and provide a safe cover and avoids atmospheric oxidation. The main functions are:

(1): Lubrication of the strand.(2): To transfer heat from strand to the mould wall.(3): Thermally insulate the top steel surface to minimize heat

loss.(4): Protect liquid steel from atmospheric oxidation.(5): Absorb non-metallic inclusions rising to the surface.

The mould fluxes may have the following composition:(i) CaO = 25-45%, Na2O = 1-20%, BaO = 0-10%, SiO2 = 20-50%,

K2O = 0-5%, Li2O = 0-4%.(ii) Al2O3 = 0-10%, FeO = 0-5%, B2O3 = 0-10%, TiO2 = 0-5%,

MgO = 0-10%, F = 4-10%.



Bhubaneswar, INDIA

Mould lubrication: The flux powders are supplied in various forms such as:

(1): Fly ash based powders. (2): Synthetic mixtures.(3): Pre-fused and ground powders.(4): Pre-fused and granulated powders. The granulated powders

are specifically suitable for automatic feeding. It is also dust free.

The most important property of interest of mould flux powders is mainly its viscosity when it melts in the steel surface in the mould. It must have corresponding liquidus temperature. Its properties should not substantially alter even on absorbing the alumina inclusions. The molten layer of the flux gets solidified at the mould wall in the form of a ring and moves down forming a protective ring.

Powder consumption: 0.3-0.6 kg/m2 of cast surface of steel.



Bhubaneswar, INDIA

Secondary cooling: The initial, or, primary cooling is accomplished in the mould. Further cooling to solidify the remaining core is called ‘secondary cooling’ and is accomplished by quenching with high pressure water sprays. (a) The sprays are directed from all sides.(b) The length is ~50 times of the minimum strand dimension. Ex: for 100 mm sq billet, the length is ~6m.(c) As the ingot emerges from the mould it is positioned with the help of several guide rolls (spray-nozzles placed in-between) held horizontally from all sides of the ingot. This is known as ‘roller apron’ which supports the ingot (i) to maintain its shape, (ii) prevents bulging and (iii) helps building without causing cracking. (d) The secondary cooling efficiency depends on (i) cooling water velocity, (ii) temperature, (iii) direction of spray, (iv) water quality and (v) scale formation.

Now-a-days, a mist of water and air spray is used.



Bhubaneswar, INDIA

Operation sequence: The operation sequence of a con-casting machine is:

(1): The machine is ready with the dummy bar in place.

(2): The teeming ladle is positioned, pre-heated and temperature of molten steel is measured.

(3): The tundish is pre-heated, the metal collected to a level and then poured to the mould.

(4): As the level of steel in mould riches a pre-determined level, the reciprocation of mould begins, dummy bar withdrawn slowly.

(5): Secondary cooling spays put on as dummy bar moves down.

(6): The lubricating oil is turned on along with start of teeming.

(7): Speed of withdrawal is adjusted to the pre-determined value.



Bhubaneswar, INDIA

Output:The output of casting varies with the cross-section being cast and the number of strands being used simultaneously. The machines can cast a product as small as 50 mm sq billet or as high as 2 m X 0.3 m size slab. The following table presents some data on rate of production from a single strand:


Product size, m Casting speed, m/min Product,~t/hr

0.05 x 0.050.10 x 0.100.15 x 0.150.20 x 0.201.25 x 1.151.50 x 0.201.75 x 0.25

5-62.5-31.5-2

0.9-1.20.8-1.20.7-0.80.5-0.7

712162180

105130


Bhubaneswar, INDIA

Yield:The slab, bloom, or billet yield from liquid steel is much higher in continuous casting than in conventional casting. A comparison is given below. (Bracketed values are for killed steel, where the advantage is more):


Conventional,~%

Con-cast, ~ % Gain, %

Liquid steelIngotsSlabsBloomsBillets

10096(95)85(84)83(76)81(71)

100100(100)

95(95)96(95)96(96)

-4(5)

10(11)13(19)15(25)


Bhubaneswar, INDIA

Process control: The process variables in a continuous casting operation are:

(1): Mould stroke (linear movement of mould): 5-10 mm range.

(2): Frequency of the stroke: 150-200/min.

(3): Casting speed: 1-7 m/min.(4): Negative strip time: 0.1-0.2 sec.(5): Total cycle time: 0.3-0.4 sec.(6): Positive strip time: 0.15-0.3 % of cycle time.(7): Oscillation depth: 0.11-0.18 mm.(8): Mould oscillation: 0.015 to max 1.0(9): casting powder viscosity: 0.10-0.20 Pas.(10): Powder consumption: 0.17-0.40 kg/m2.



Bhubaneswar, INDIA

Defects in continuous cast products: The common defects in con-cast products are listed below:

(1): Alumina streaks. (2): Edge cracks. 3): Longitudinal cracks.

(4): Primary scales. (5): Segregation. (6): Slivers.(7): Roll marks, roll peel-off, rolled in scale, roll wear.

(8): Random surface defects arising due to fluctuations of the surface level ‘telescopicity’ and stagger.

(9): Variation in width (curly edges).(10): Non-uniform thickness and flatness (ripples in both

directions).(11): Non-uniform microstructure in either direction.(12): Oscillation marks – the slag is trapped in this.(13): Slag particles sticking to surface.



Bhubaneswar, INDIA

CONTINUOUS CONTINUOUS CASTINGCASTING

Phenomena in the mould region during casting


Bhubaneswar, INDIA

Defects in continuous cast products: The common defects in con-cast products are listed below:

(1): Alumina streaks. (2): Edge cracks. (3): Longitudinal cracks.

(4): Primary scales. (5): Segregation. (6): Slivers.(7): Roll marks, roll peel-off, rolled in scale, roll wear.

(8): Random surface defects arising due to fluctuations of the surface level ‘telescopicity’ and stagger.

(9): Variation in width (curly edges).(10): Non-uniform thickness and flatness (ripples in both

directions).(11): Non-uniform microstructure in either direction.(12): Oscillation marks – the slag is trapped in this.(13): Slag particles sticking to surface.



Bhubaneswar, INDIA

Defects in continuous cast products(cont):

(A): Defects arising from lubrication – H pick up from oil.(B): Defects arising from deoxidization practice – alumina

inclusion. (C): Calcium treatment.(D): Electromagnetic stirring.(E): Prevention of oxidation during casting – use of protective

cover on liquid metal in tundish, ladle and on metal stream. Use of protective cover when it flows from tundish to mould.

(F): Control sensors – sensor using (i) radioactive radiation, (ii) electromagnetism, and (iii) eddy current mould level indicator.



Bhubaneswar, INDIA

Recent trends in continuous casting: (A): Continuous – continuous casting.(B): Combination castor. (C): Near net shape casting – compact strip production.

Comparison of processes for producing thin strip steel


Parameter Caststrip Thin slab Thick slab

Cast thickness, mmCasting speed, m/minAve mould flux, m.wt/m2

Total solidification time, secAve shell cooling rate, 0C/sec

1.68014

0.151700

506

2.54560

2202

1.01070

12


Bhubaneswar, INDIA


Progressive changes in the continuous casting machine

(a) Standard curved mould curvilinear withdrawal

Relatively thinner slab caster

Future near net shape caster

RKP-NIT-MM 325-E

Documents

Transcript of RKP-NIT-MM 325-E