Inclusion control for clean steel

SANTOSH KUMARMGR(SMS)

NINL

Inclusion Control for Clean Steel

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Contents1. Introduction

2. Inclusion assessment

3. Inclusion Source & Control

4. Inclusion Modification

5. Conclusion

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1. IntroductionSteel cleanliness is the one unifying theme in all steel

plants as problems in steel cleanliness can lead to internal rejects or customer dissatisfaction with steel products. Thus all steel plants are continually attempting to improve their practices to produce more consistent products.

Two main keys to the production of quality steel products

Chemistry and Inclusion control

These results can only be reached by a strict control of process 3

Non-Metallic Inclusion: Non-metallic inclusions are chemical compounds of metals (Fe, Mn, Al, Si, Ca) with non-metals (O, S, C, H, N). Non-metallic inclusions form separate phases.

Clean Steel: Clean Steel refers to steel which is free from inclusions and Level of cleanliness of steel is determined by no. of inclusions per ton of steel.

1.1 Definition

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The study of non-metallic inclusion is important for two reason

1) The first is their influence on the properties and the quality of steel products. This is a significant aspect from the point of view of steel product users, who have to take into account the presence of inclusions in evaluating the material behaviour in working condition.

2) The second reason is that the study of inclusion allows to estimate techniques and chemical reactions in steel refining.

1.1 Reason of study of inclusion

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1.2 Influence of Inclusion on Steel Properties

Despite of small content of non-metallic inclusions in steel (0.01-0.02%) they exert significant effect on the steel properties such as:

- Tensile strength

- Deformability (ductility)

- Toughness

- Fatigue strength

- corrosion resistance

- Weldability

- Polishability

- Machinability6

Hydrogen

Carbon

NitrogenOxygen

Phosphorus

Sulfur

Non metallic elements

Electromagnetic

properties

Deep drawing

Weldability

Toughness

Internal soundnes

s

Surface defects

Anisotropy

Fatigue

Bending

1.2 Influence of Inclusion on Steel Properties

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1.3 Type of Inclusion

Oxides: FeO, Al2O3, SiO2, MnO, Cr2O3 etc.

Sulfides: FeS, MnS, CaS, MgS, Ce2S3 etc.

Oxysulfides: MnS*MnO, Al2O3*CaS, FeS*FeO etc.

Carbides: Fe3C, WC, Cr3C2, Mn3C, Fe3W3C etc.

Nitrides: TiN, AlN, VN, BN etc.Carbonitrides: Titanium

carbonitrides, vanadium carbonitrides, niobium carbonitrides etc.

Phosphides: Fe3P, Fe2P, Mn5P2

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Micro Inclusion: 1-100 m m Beneficial as they restrict grain growth, increase yield

strength and hardness Act as a nuclei for precipitation of carbides and nitrides

Macro Inclusion : >100 m m Harmful in nature so must be removed

1.4 Size of Inclusion

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Product Allowed impurities (in ppm)

Allowed size (mm)

Automotive Sheet & Deep drawing sheet

C<30, N<30, TO<20 100

Drawn and Ironed Cans

C<30, N<40,TO<20 20

Tire Cord H<2, N<40, TO<15 10

Ball Bearings [Ti] < 15, TO<10 15

Line pipe S<10, N<50, TO<30 100

Wires N<40, TO<15 20

Heavy plate H<2, N<40, TO<20 13

1.4 Requirements for Clean Steels

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1.5 Morphology of Inclusion

Globular shapePlatelet shaped Dendrite shaped Polyhedral Shaped

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SEM image of an inclusion observed in the duplex stainless steel after calcium treatment


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(A): inclusion containing Si and Cr. (B): inclusion containing Al and Cr.Formation Mechanism of Non-Metallic Inclusions in Stainless Steel


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a) As-polished (2-dimensional) steel sample showing Al2O3 dendrite

b) steel sample showing the same Al2O3 dendrite(SEM image) 14

Thermal Expansion:

MnS, CaS etc. have a thermal expansion greater than steel matrix.

- On heating steel, void or parting of the matrix can occur. The void act as crack

Al2O3, SiO2, CaO.Al2O3 etc have a thermal expansion smaller than steel matrix

- On heating internal stresses developed

1.6 Properties of Inclusion

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Density & Melting Point:


Compound of Inclusion

Melting Point(oC) Density at 20oC (g/cm3)

FeO 1369 5.8

MnO 1785 5.5

SiO2 1710 2.2-2.6

Al2O3 2050 4.0

CrO2 2280 5.0

TiO2 1825 4.2

ZrO2 2700 5.75

(FeO)2.SiO2 1205 4.35

FeS 988 4.6

MnS 1620 4.04

MgO 2800 3.58 16

Plastic Deformability: Calcium aluminates and Al2O3 inclusions in steel are un-

deformable at temperatures of interest in steelmaking Spinel type double oxide AOB2O3 are deformable at

temperature greater than 1200oC(where A is Ca,Fe(l),Mg, Mn & B is Al, Cr )

Silicate are deformable at higher temperature. FeO, MnO are plastic at room temp but gradually lose

plasticity above 400oC Mns is highly deformable at 1000oC but slightly less

deformable above 1000oC Pure silica is not deformable upto 1300oC


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Inclusion counts are performed to assess their shape, quality and distribution to assess about the cleanliness of steel

2.0 Inclusion Assessment

• Inclusion counting by image analysis

Inclusion Analysis

Sample preparation

Qualitative Assessment Quantitative Assessment

• Dissolution of matrix by SPEED method

• Image acquisition by SEM – Back scattered electron mode

• Inclusion species and morphology study by SEM and EDS

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Images acquired using (a) optical microscopy, (b) laser confocal microscopy,(c) SEM (secondary electron mode) and (d) SEM (backscattered electron mode)


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Glassy Al2O3 (globular) inclusions found in 1018S furnace tap sample


Glassy Al2O3 (plate) inclusions found in 1018S ladle sample

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Oxide inclusions found in ladle sample: alumina

Oxide inclusion found in billet sample: alumina dendrites


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Oxide inclusions found in A529 ladle sample: a) alumina and galaxite (G)b) alumina cluster


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Mechanism of inclusion formation:

Indigenous inclusions are formed in liquid, solidified or solid steel as a result of chemical reactions (deoxidation, desulfurization) between the elements dissolved in steel.

Exogenous inclusions are derived from external sources such as furnace refractories, ladle lining, mold materials etc.

3.0 Source of Inclusion Formation

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Source of Inclusion:

I. Primary inclusions: generated during deoxidation reaction

II. Secondary inclusions: generated due to equilibrium shift as temperature decreases during vessel transfer, such as tapping and teeming operations

III. Tertiary inclusions: generated during the process of solidification, usually characterized by rapid cooling

IV. Quaternary inclusions: generated during solid state phase transformation, which causes changes in solubility limits of various constituents

3.1 Inclusion Source & Control

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There are three stages of inclusions formation:

1. NucleationNuclei formed as a result of super-saturation of the solution with the solutes The nucleation process is determined by surface tension on the boundary inclusion-liquid steel.The nucleation process is much easier in the presence of other phase (other inclusions) in the melt.

2. GrowthGrowth of a separate inclusion continues until the chemical equilibrium is achieved (no super-saturation).very slow process

3. Coalescence and agglomerationMotion of the molten steel due to thermal convection or forced stirring causes collisions of the inclusions, which may result in their coalescence (merging of liquid inclusions) or agglomeration (merging of solid inclusions)

3.2 Inclusion Formation

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Besides of the shape of non-metallic inclusions their distribution throughout the steel grain structure is very important factor determining mechanical properties of the steel.Homogeneous distribution of small inclusions is the most

desirable type of distribution. Location of inclusions along the grain boundaries is

undesirable since this type of distribution weakens the metal.Clusters of inclusions are also unfavorable since they may

result in local drop of mechanical properties such as toughness and fatigue strength.

Distribution of non-metallic inclusions may change as a result of metal forming (eg. Rolling).

3.3 Distribution of Inclusion

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Al2O3 (inclusion in steel) SEM image

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Inclusion can be controlled at:a) During liquid steel processing stageb) During solid state processing

3.4 Control of Inclusion

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i) Control of inclusion during tapping of steel Carry-over slag to be minimized

- Carry over of 1 kg FeO in slag decrease Al by 0.286 kg , which in turn forms 0.51 kgAl2O3

- No. of inclusion are 240 per kg of carry over FeO of slag


ii) Control of inclusion during treatment of steel Stirring of steel bath accelerate the inclusion float to surface

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iii) Control of inclusion during teeming of steel Shrouding of molten steel stream in order to avoid re-oxidation. Proper selection of tundish flux Segregation during solidification to be avoided


iv) Control of inclusion during Solid state processing Working temp range 800-1200oC Inert atmosphere to avoid oxidation

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Depends on application, Inclusion Modification Technique is based on design of inclusions so as to minimize their harmful effects on the product properties.

Uniformly dispersion of inclusion in the matrix

4.0 Inclusion Modification

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It should have high chemical affinity for the inclusion

It should be able to modify the composition so that it becomes liquid.

It should be able to modify the shape i.e sharp edges and corner of inclusion to spherical.

4.1 Requirement for Inclusion Modification

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• Mainly Al2O3 and MnS inclusions are modified

• Al2O3 inclusions are solid at casting temperature & brittle in nature. Therefore clog the nozzle at continuous casting and breaks on deformation

• MnS inclusion in steel on deformation becomes stringer type.

• Ca is used widely to modify inclusion• Solubility of C in steel is 320 ppm at 1600oC• Density of Ca: 1.55 g/cm3

• Melting Temp of Ca: 1439oC• Form vapour at steel temperature 1600oC

4.2 Ca-treatment for inclusion modification

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• CaO-Al2O3 Equilibrium Phase diagram


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Compound Melting Temp (oC)

Ca/Al

Al2O3 2050 --CaO.2 Al2O3 1727 0.37

CaO. Al2O3 1595 0.74

12CaO.7Al2O3 1400 1.27

3CaO.Al2O3 1527 2.22

CaS 2000 --MnS 1620 --

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• Ca first modify the oxide inclusion

• Thermodynamically it is easier to form CaO.2Al2O3 Then converted to CaO.Al2O3 and finally liquid calcium aluminate rich in CaO

• Then Ca desulphurise to very low levels.

• When Ca content reach a certain level (~34ppm), CaS precipitation begins

• This will result in precipitation of CaS which forms a duplex inclusion in which CaS-MnS ring surrounds calcium aluminate core. This type of inclusion is spherical and does not elongate.


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Ca treatment is commonly used to control the shape and composition of both oxydes and sulphides inclusion in Al-Killed steel. The Ca additions reacts with solid Al2O3 inclusion. Generally Ca. aluminates of lower melting points. Some of the Ca may also react with dissolved sulpher resulting in the formation of Ca or Ca-Mn sulphide inclusion. Problem of nozzle clogging are often related to micro-inclusion composite –either aluminate with a high Al2O3 or CaS inclusion are solid at steel melting temperature promoting nozzle blockage.


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During Ca treatment of Al-killed steel, some Ca dissolved in the steel and subsequently react with solid AL2O3 inclusion to form calcium silicate. As the addition of Ca proceeds, the inclusion become increasing rich in CaO and their liquidus temperature decreases. Some of the added calcium may combined with sulpher to produce Ca-Mn sulphide.

When Ca content reached a certen level ~ 34 ppm, CaS precipitation begins


Oxide inclusions found in ladle tap sample: calcium aluminate


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It not always important to remove the inclusion from steel, however , the bigger size inclusion are to be removed.

Smaller size inclusion is not all the time required that can be removed, however, if those inclusions can be modified in terms of their melting point, or in terms of their sharp edges or corner edges modified to spherical globule, then it will be good. From application point of view , they will not have a harmful effect.

5. Conclusion

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THANK YOU

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