Clean Steel

LECTURE #4
CLEAN STEEL
WHAT IS CLEAN STEEL?
Clean steel refers to steel free from inclusions but
practically, it is not possible to produce steel without
any inclusions.
CHARACTERISTICS OF CLEAN
STEEL:
Low total oxygen content
Low level of non-metallic inclusions (<50µ)
No existence of macro inclusions (>50µ)
Inclusion morphology (spherical)
Low solute elements: O, N, P, S & H
Low residual elements: Cu, Pb, Zn ,etc
TYPES OF INCLUSIONS:
Oxides:
FeO ,SiO2 ,Al2O3 ,etc or complex oxides.
Sulphides
FeS, MnS, MgS, etc or compounds
Nitrides
TiN, AlN, ZrN, etc or compounds
Phosohides
Fe3P, Mn5P2, etc

DISADVANTAGES OF HIGH LEVEL OF NMI &
MACRO INCLUSIONS
They deteriorate the following properties:
o Impact strength
o Fatigue strength
o Formability
o Drawability
o Weldability especially (ERW)

o Corrosion resistance
o Resistance to HIC (hydrogen induced cracking)
CLEANLINESS REQUIREMENTS FOR VARIOUS STEEL GRADES:
Critical Inclusion Maximum Allowed Impurity
Application Key Property
Size,µm content ,ppm
C ≤ 30
Deep drawing
-Flange cracking ≤ 20 N ≤ 40
Cans sheets
T.O ≤ 20
Automotive and
-Rupture C ≤ 30
deep drawing ≤ 50
-Cracking N ≤ 30
sheets
S ≤ 10
Sour gas pipes -HIC Shape control P ≤ 50
H ≤ 1.5
S ≤ 30
Shape control N ≤ 50
Line pipes -SCC
≤ 100 T.O ≤ 30
H ≤ 1.5
T.O ≤ 10
Bearing -Fatigue cracking ≤ 15 Ti<15(TiO2,TiN:non-deformable)
H ≤2
≤ 10
Tire cord -Rupture N ≤ 40
Shape control
T.O ≤ 15
Spring wire -Fatigue cracking Shape control T.O ≤ 20
C ≤ 30
-Rupture
IF Steel Shape control N ≤ 40
-Cracking
T.O ≤ 40
PROPERTIES OF INCLUSIONS
Thermal expansion
o Inclusions like MnS ,CaS have thermal expansion greater than

steel matrix.
On heating steel voids can occur and act as cracks.

o Inclusions like Al2O3, SiO2 & CaO.Al2O3 have thermal
expansion smaller than steel matrix.
On heating steel internal stresses can occur.

 Density & Melting Point
Inclusion Melting point Density at
200C,gm/cm3
FeO 1369 5.8
MnO 1785 5.5
SiO2 1710 2.4
Al2O3 2050 4.0
Cr2O3 2280 5.0
TiO2 1825 4.2
ZrO2 2700 5.75
(FeO)2SiO2 1205 4.35
FeS 988 4.6
MnS 1620 4.04
MgO 2800 ----
12CaO.7Al2O3* 1400 ----
Inclusions like Al2O3, TiO2, MgO are solids during steelmaking process.
Plastic deformability
The plastic deformability of an inclusion will govern any change in its shape under
the action of external forces and will determine the amplitude of stress
concentration.
Brittle inclusion are dangerous as they may crack and cause fracture.
 CaO.Al2O3 and Al2O3 are undeformable at temperature of interest in steelmaking
 Spinal type oxides AOB2O3 are deformable at temperature > 12000C where
A: Ca, Fe, Mg, Mn
B: Al, Cr
 FeO, MnO are plastic at room temperature but lose plasticity above 4000C.
 MnS is highly deformable at 10000C but slightly less deformable above 10000C
 Pure SiO2 is not deformable up to 13000C

SOURCES OF NON METALLIC & MACRO
INCLUSIONS:
Sources of Inclusions
Endogenous Inclusions Exogenous inclusions
Deoxidation products Reoxidation Inclusions from refractories Slag entrapment

or linning
Slag & linning
reactions
Too high argon
Reaction of soluble oxygen & Reaction between Asol &
deoxidizers Al & Si
stirring rate
FeO, MnO , SiO2
Failure of Filling TD in the

Exposure to atmosphere weir or dam presence of slag
(poor sealing)
Erosion during
Low bath depth in
casting tundish (vortex
Reoxidation can cause formation)
nozzle clogging
High mold level
fluctuation
It is not recommended too much flow

towards top surface or stagnant surface
CHARACTERISTICS OF EXOGENOUS
INCLUSIONS
Large size
Multiphase
Irregular shape
Small number
Often found near the surface
More harmful to steel properties
CLEAN STEEL PRACTICE
 Minimize slag carryover from EAF at tapping (hot heel: ~ 30 ton)
 Slag treatment to reduce FeO + MnO (FeO + MnO < 1%)
 Ca treatment at high feeding speed
 Minimum 7 minutes rinsing with low argon flow rate after Ca-treatment
 Control mold level within 3mm
 Control tundish super heat to be 10 ~ 250C
NB
Too low super heat doesn’t allow for large inclusions to float
Ca-TREATMENT
Purpose
To modify inclusion shapes to avoid alumina (Al2O3)
build up on the internal surface of the submerged entry
nozzle (SEN) causing clogging.Al2O3 is very
detrimental to machinability
Steel desulfurization
Steel deoxidation
Ca-TREATMENT
Ca treatment pattern
 Al must be adjusted before Ca-treatment
 Ca-treatment according to the type of Ca-wire used
 Keeping 10 ~ 15 minutes soft bubbling between last Al addition and Ca-treatment start
 minutes soft bubbling for inclusion floating after Ca injection
 Aim of Alsol/Ca (it is recomnded to be < 15%)
NB
 Check slag surface before Ca injection and use pipe to open a hole in case of hard slag
surface
 Use suitable Ca-wire in case of galvanized steel & non galvanized steel
 After Ca injection, sample is taken after 2 minutes soft bubbling
 Ca-wire speed is > 150m/min
Ca-TREATMENT
 Reactions
 [Ca] + [O] = (CaO) (1)

 [Ca] + [S] = (CaS) (2)
 (Al2O3) +3(CaS) = 3(CaO) + 2[Al] + 3[S] (3)
 [S] + (CaO) = (CaS) + [O] (4)
 2[Al] + 3[O] = (Al2O3) (5)
 [Ca] + (X+(1/3))(Al2O3) = (2/3) [Al] (6)

Ca-TREATMENT
Calcium aluminates retained in liquid suppress the formation of MnS
stringers during solidification of steel. The modification of shape of
inclusions and the low melting point of these inclusions have the
following advantages:
 Improve steel castability
(minimize nozzle clogging).

 Minimize susceptibility steel
to heat-affected zone cracking.

 Minimize susceptibility of HSLA steels
to HIC in sour gas/oil environment.

 Increase impact energy
Binary diagram of CaO & Al2O3
Ca-TREATMENT
The morphology of Al2O3 is dendritic and it does not float easily so Ca is added to
turn the dendritic morphology into spherical one to facilitate its floating to the slag
layer.
Modification of inclusions by Ca treatment

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LECTURE #4

FeO ,SiO2 ,Al2O3 ,etc or complex oxides.

FeS, MnS, MgS, etc or compounds

TiN, AlN, ZrN, etc or compounds

Fe3P, Mn5P2, etc

o Weldability especially (ERW)

o Inclusions like MnS ,CaS have thermal expansion greater than

On heating steel voids can occur and act as cracks.

On heating steel internal stresses can occur.

 CaO.Al2O3 and Al2O3 are undeformable at temperature of interest in steelmaking

A: Ca, Fe, Mg, Mn

 Pure SiO2 is not deformable up to 13000C

Endogenous Inclusions Exogenous inclusions

Deoxidation products Reoxidation Inclusions from refractories Slag entrapment

Failure of Filling TD in the

It is not recommended too much flow

 Aim of Alsol/Ca (it is recomnded to be < 15%)

 [Ca] + [O] = (CaO) (1)

 (Al2O3) +3(CaS) = 3(CaO) + 2[Al] + 3[S] (3)

 [S] + (CaO) = (CaS) + [O] (4)

 2[Al] + 3[O] = (Al2O3) (5)

 [Ca] + (X+(1/3))(Al2O3) = (2/3) [Al] (6)

stringers during solidification of steel. The modification of shape of

(minimize nozzle clogging).

to heat-affected zone cracking.

to HIC in sour gas/oil environment.

Modification of inclusions by Ca treatment

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