This document discusses inclusion control for clean steel production. It defines inclusions as non-metallic compounds that form separate phases in steel. Strict inclusion control is important for producing quality steel products. Inclusions are assessed and controlled by examining their source, shape, composition and distribution. Common inclusions include oxides, sulfides, and carbides. Modification techniques aim to make inclusions less harmful by modifying their shape, composition and dispersion in the steel matrix. Calcium additions are often used to modify alumina and manganese sulfide inclusions. Proper inclusion control is important at all stages of steelmaking and processing to achieve clean steel.

1. Inclusion Control for Clean
Steel
SANTOSH KUMAR
MGR(SMS)
NINL
1

2. Contents
1.
2.
3.
4.
5.
Introduction
Inclusion assessment
Inclusion Source & Control
Inclusion Modification
Conclusion
2

3.  Steel 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

4.  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.
4

5. 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.
5

6. 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
- Machinability
6

7. Non metallic elements
Hydrogen
Electromagnet
ic
properties
Toughnes
s
Weldability
Carbon
Nitrogen
Oxygen
Phosphor
us
Sulfur
Internal
soundness
Deep drawing
Surface defects
Fatigue
Anisotropy
Bending
7

8.  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.
8

9.  Micro Inclusion: 1-100 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
 Harmful in nature so must be removed
9

10. Product
Allowed impurities
(in ppm)
C<30, N<30, TO<20
Allowed size ( m)
Drawn and Ironed
Cans
C<30, N<40,TO<20
20
Tire Cord
H<2, N<40, TO<15
10
[Ti] < 15, TO<10
15
S<10, N<50, TO<30
100
N<40, TO<15
20
H<2, N<40, TO<20
13
Automotive Sheet &
Deep drawing sheet
Ball Bearings
Line pipe
Wires
Heavy plate
100
10

11.  Globular shape
 Platelet shaped
 Dendrite shaped
 Polyhedral Shaped
11

12. SEM image of an inclusion observed
in the duplex stainless steel after
calcium treatment
12

13. (A): inclusion containing Si and Cr. (B): inclusion containing Al and
Cr.
Formation Mechanism of Non-Metallic Inclusions in
13

14. a) As-polished (2-dimensional) steel sample showing Al2O3
dendrite
b) steel sample showing the same Al2O3 dendrite(SEM
image)
14

15.  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
15

16.  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

17.  Plastic Deformability:
 Calcium aluminates and Al2O3 inclusions in steel are undeformable 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
17

18.  Inclusion counts are performed to assess their
shape, quality and distribution to assess about the
cleanliness of steel
Inclusion Analysis
Sample preparation
Qualitative Assessment
• Dissolution of matrix by
SPEED method
• Inclusion species and
morphology study by
SEM and EDS
Quantitative Assessment
• Image acquisition by
SEM –
• Back scattered electron
Inclusion counting
mode
by
image analysis
18

19. Images acquired using (a) optical microscopy, (b) laser confocal
19
microscopy,

20. Glassy Al2O3 (globular)
inclusions found in 1018S
Glassy Al2O3 (plate)
inclusions found in 1018S
20

21. Oxide inclusions found in
ladle sample: alumina
Oxide inclusion found in
billet sample: alumina
dendrites
21

22. Oxide inclusions found in A529 ladle sample: a) alumina and
galaxite (G)
22

23. 
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.
23

24.  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
24

25. There are three stages of inclusions formation:
1. Nucleation
Nuclei 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. Growth
Growth of a separate inclusion continues until the chemical
equilibrium is achieved (no super-saturation).
very slow process
3. Coalescence and agglomeration
Motion 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)
25

26.  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).
26

27. 27

28. Al2O3 (inclusion in steel) SEM image
28

29.  Inclusion can be controlled at:
a) During liquid steel processing stage
b) During solid state processing
29

30. 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
30

31. iii) Control of inclusion during teeming of steel
 Shrouding of molten steel stream in order to avoid reoxidation.
 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
31

32.  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
32

33.  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.
33

34. •
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
34

35. •
CaO-Al2O3 Equilibrium Phase diagram
35

36. Compound
Al2O3
CaO.2 Al2O3
CaO. Al2O3
12CaO.7Al2O3
3CaO.Al2O3
CaS
MnS
Melting Temp (oC)
2050
1727
1595
1400
1527
2000
1620
Ca/Al
-0.37
0.74
1.27
2.22
---
36

37. 37

38. •
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.
38

39.  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.
39

40.  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
40

41. Oxide inclusions found in ladle tap sample: calcium
aluminate
41

42.  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
42
effect.

43. 43