J. BURJA et al.: NITROGEN AND NITRIDE NON-METALLIC INCLUSIONS IN STEEL
919–928
NITROGEN AND NITRIDE NON-METALLIC INCLUSIONS IN
STEEL
DU[IK IN NITRIDNI NEKOVINSKI VKLJU^KI V JEKLU
Jaka Burja
1
, Mitja Kole`nik
2
, [pela @uperl
3
, Grega Klan~nik
4,5
1
Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
2
SIJ Metal Ravne d. o. o., Koro{ka cesta 14, 2390 Ravne na Koro{kem, Slovenia
3
[tore Steel d.o.o., @elezarska cesta 3, 3220 [tore, Slovenia
4
SIJ Acroni d.o.o., Cesta Borisa Kidri~a 44, 4270 Jesenice, Slovenia
5
RCJ d.o.o., Cesta Franceta Pre{erna 61, 4270 Jesenice, Slovenia
Prejem rokopisa – received: 2019-10-15; sprejem za objavo – accepted for publication: 2019-11-02
doi:10.17222/mit.2019.247
The paper deals with the formation of coarse nitride non-metallic inclusions in steel. The thermodynamic calculations for the
determination of the equilibrium solubility of nitrogen in the steel melt at a given temperature and chemical composition are
presented. Some of the negative effects of excessive nitrogen contents, like blow holes, strain aging and, most importantly,
nitride non-metallic inclusion formation are discussed. Five types of the coarse nitride non-metallic inclusions that occur in
industrial practice are presented: aluminium, boron, niobium, titanium, and zirconium nitrides. Examples of the negative effect
of coarse nitrides from case studies are also shown. The basic thermodynamic tools and values of the parameters for calculating
the nitride precipitation in the steel melt are given in the paper.
Keywords: non-metallic inclusions, steelmaking, nitrides, nitrogen in steel
V ~lanku obravnavamo tvorbo grobih nitridnih nekovinskih vklju~kov v jeklu. Predstavljeni so termodinamski izra~uni za
dolo~anje topnosti du{ika v jekleni talini pri poljubni temperaturi in sestavi jeklene taline. Prikazali smo nekatere negativne
vplive previsoke vsebnosti du{ika v jeklu, kot so: nastanek plinske poroznosti, staranje in predvsem nastanek nitridnih
nekovinskih vklju~kov. Predstavili smo pet tipov grobih nitridov, ki se pojavljajo v industrijski praksi. To so aluminijevi, borovi,
niobijevi, titanovi in cirkonijevi nitridi. Prikazali smo tudi negativni vpliv grobih nitridov na lastnosti jekel. V ~lanku smo podali
osnovna termodinamska orodja in vrednosti parametrov za izra~un izlo~anja nitridov v jekleni talini.
Klju~ne besede: nekovinski vklju~ki, jeklarstvo, nitridi, du{ik v jeklu
1 INTRODUCTION
The increasing demands for high-quality products
have made the steelmaker increasingly aware of the steel
"cleanliness" requirements. The mechanical properties of
steel are affected by the volume fraction, size, distri-
bution, composition, and morphology of non-metallic
inclusions and precipitates, which can act as stress
raisers.
1,2
For example, fatigue properties, ductility and
fracture toughness are known to be decreased by non-
metallic inclusions, especially in higher-strength,
lower-ductility alloys.
3–7
The most important parameters influencing clean
steel production are the steel composition, the slag com-
position, the refractory lining, the melt processing, and
the casting practice.
1
The steel composition includes oxygen, nitrogen and
sulphur levels in combination with oxide forming (Al,
Ca, Si, Mg …), nitride forming (Ti, Nb …) and sulphide
forming (Ca, Mn …) elements. The slag composition in-
cludes basicity (CaO:SiO
2
) and some oxides, like Al
2
O
3
,
MgO and FeO+MnO (slag carryover). The refractory
lining needs to be resistant to slag erosion (minimal
silica content); the refractory is the main source of MgO
(causes spinel formation). The melt processing is of
paramount importance to clean steel production. This
includes the tapping practice (reoxidation, slag carry-
over), alloying (including deoxidation and modification)
where the addition timing and amounts are important.
Other secondary refining processes like melt stirring, de-
sulphurisation, vacuuming (decreases oxygen, nitrogen
and sulphur at once) decrease the amount of non-metallic
inclusions. The casting practice is the final factor and it
can have a detrimental effect. Firstly, there is an
important difference between ingot (lower cleanliness)
and continuous casting (higher cleanliness), then there
are also accompanying processes and materials like
electromagnetic mould stirring, argon shrouding of
nozzles and mould flux that also greatly affect the final
steel cleanliness.
The content of non-metallic inclusions is generally
connected to the amount of total oxygen and sulphur in
the steel, as oxides and sulphides commonly form the
bulk of the non-metallic inclusions. The total oxygen in
the steel is the sum of the free oxygen (dissolved
oxygen) and the oxygen that is chemically bound in
compounds (non-metallic inclusions). Free oxygen, or
"active" oxygen, can be measured in molten steel using
Materiali in tehnologije / Materials and technology 53 (2019) 6, 919–928 919
UDK 620.1:669.058.4:669.14:669.786 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(6)919(2019)
*Corresponding author's e-mail:
jaka.burja@imt.si (Jaka Burja)

oxygen sensors (usually along with temperature
measurements). The free oxygen is controlled by the
equilibrium thermodynamics with deoxidation elements,
such as aluminium and is usually low (3-5 μg/g).
Because the free oxygen does not vary much, the total
oxygen is a reasonable indirect measure of the total
amount of oxide inclusions in the steel.
1
Low oxygen and
sulphur levels must be achieved to ensure low non-
metallic inclusion contents. This can be achieved with
steelmaking practices, by properly refining the steel melt
during the ladle treatment, using sufficient degassing
times, vacuuming, etc. The presence of elements like
aluminium, calcium, silicon, manganese, rare-earth me-
tals, magnesium and zirconium has a profound effect on
the non-metallic inclusion formation and related size,
number, chemistry and morphology.
8–13
Nitrogen, on the other hand, is often an overseen
factor in the prediction of the non-metallic content.
Specialized test methods cover a number of recognized
procedures for determining the non-metallic inclusion
content of steel, these methods are primarily intended for
rating oxide and sulphide inclusions. But constituents
such as carbides, nitrides, and carbonitrides may be rated
by adopting some of the microscopic methods described
in ASTM E45.
14
This is probably due to the fact that
only special alloying elements form nitride non-metallic
inclusions. The fact is that all steels contain some
nitrogen, which can enter the steel as an impurity or as
an intentional alloying addition. Intentional nitrogen
alloying includes stirring the melt with nitrogen gas and
adding ferro-alloys with increased nitrogen, such as
ferrochromium nitre (4 % N) and nitrovan vanadium
(16 % N). However, numerous other less-desired sources
of nitrogen (nitrogen pickup), and sometimes other gases
(oxygen, hydrogen), exist during the melting.
• Nitrogen is present in the charge/feed materials.
• Nitrogen in the stirring gases (lance and bottom
stirring).
• Nitrogen pick up from ladle addition such as
carburizers and ferro-alloys,
• Nitrogen is absorbed from the atmosphere during the
tapping of steel or excessive stirring.
The [N] pickup in steel melt is also related to the
[O]
total
increase and also lower steel cleanliness.
15
The basic thermodynamics for the calculation of
equilibrium nitrogen solubility (phase equilibria) in the
steel melt at different steel-melt compositions are
presented and discussed in this paper along with nitride
non-metallic inclusion precipitation in the melt. Some
practical examples of coarse cuboidal nitride inclusions
and their effect on steel properties are also presented.
Due to limited dissolution of coarse nitrides during
solution annealing prior to hot rolling their formation
must be limited.
2 THERMODYNAMIC BACKGROUND
The dissolution of nitrogen in molten steel is ex-
pressed in Equation (1):
16
[]
1
2
NN
2(g)
= (1)
[]
K
w/ f
p
N
N
N
N
2
=
⋅ %
()
(2)
The nitrogen solubility in liquid Fe at the standard
1 atmosphere nitrogen pressure was determined by H.
Wada et al.
17
as:
lg . K
T
N
=−
430
1125 (3)
An example of the effect of partial N
2
(g) pressure and
temperature on solubility in liquid Fe is shown in
Table 1.
According to Table 1 it is obvious that different con-
cepts of steelmaking practices (like vacuum degassing)
can result in different values of nitrogen after degassing.
Table 1: Equilibrium dissolved nitrogen in Fe(l) at different N
2
pressures (1 atm to 0.001 atm) and temperatures (1723 K to 1973 K)
N
2
pressure (atm)
Temperature 1 0.1 0.01 0.001
K Equilibrium dissolved N (w/%)
1973 0.0454 0.0144 0.0045 0.0014
1923 0.0448 0.0142 0.0045 0.0014
1873 0.0442 0.0140 0.0044 0.0014
1823 0.0436 0.0138 0.0044 0.0014
1773 0.0429 0.0136 0.0043 0.0014
1723 0.0422 0.0133 0.0042 0.0013
The solubility of nitrogen in liquid steel changes with
the steel chemical composition. The constant in equation
K
N
can be modified for a given steel composition and
nitrogen partial pressure using Equation (4). While the
influence of nitrogen pressure is rather straightforward,
the influence of the chemical composition is calculated
by using first-order interaction coefficients, as shown by
Equation (5). The data for interaction coefficients is
given in Table 2. The values of the interaction para-
meters can, however, vary. Zirconium and titanium are
the elements that most strongly increase the nitrogen
solubility; however, they regularly form nitrides and can
cause non-metallic inclusion formation. In practice
chromium is the most influential, due to its high alloying
content in stainless steels. Some austenitic stainless
steels like Nitronic 50 can have up to 0.4 w/% nitrogen
along with 23 w/% Cr.
[]
lg % lg lg lg w/ K f p N
NNN
2
=−+
1
2
(4)
[]
lg % few /
x
NN
N =⋅ ∑ (5)
J. BURJA et al.: NITROGEN AND NITRIDE NON-METALLIC INCLUSIONS IN STEEL
920 Materiali in tehnologije / Materials and technology 53 (2019) 6, 919–928

Table 2: Interaction coefficients e
y
x
in dilute liquid iron for selected
elements at 1873 K
18–23
Y\X Al B C Cr H Mn Mo N
Al 0.045 0.091 0.24 –0.058
B 0.038 0.22 0.49 0.074
N –0.028 0.094 0.13 –0.047 0 –0.02 –0.011 0
Nb –0.49 –0.61 0 –0.42
Ti 0.0037 –0.165 0.055 –1.1 0.0043 –1.8
V –0.34 –0.59 –0.35
Zr –4.1
Nb Ni O S Si Ti V Zr
Al –6.6 0.03 0.0056 0.004
B –1.8 0.048 0.078
N –0.06 0.01 0.05 0.007 0.047 –0.53 –0.093 –0.63
Nb –0.83 –0.047
Ti –1.8 –0.11 0.05 0.048
V –0.97 –0.028 0.042 0.015
Zr –2.6 –0.16 0.02
A graphical representation of the influence of some
alloying elements on the nitrogen solubility is given in
Figure 1. The rule of thumb is that the nitride-forming
elements increase the solubility, while others like Ni and
Si decrease it.
3 NITROGEN IN STEEL
Generally, while the steel is liquid the nitrogen is
always present in the solution. However, the solubility of
nitrogen in liquid steel is commonly a couple of times
greater than in  -ferrite or  -austenite, as seen in Fig-
ure 2. So, when considering the effect of nitrogen
alloying its change in solubility during solidification
must be considered.
Excessive nitrogen almost always impairs the steel’s
target functional or mechanical properties. Of course,
this also depends on the steel’s chemical composition,
micro-alloying elements and the target values of carbon
and nitrogen. But generally, the solidification of steel can
result in three nitrogen-related phenomena:
• formation of blowholes;
• solidification of steel with nitrogen in interstitial
solid solution;
• precipitation of one or more nitride compounds.
3.1 Blowholes
Blowholes (porosity) are formed when the solubility
of the gases dissolved in liquid steel is locally exceeded.
In this case the thermodynamic driving force tends to
reduce the dissolved gas content by forming gas bubbles.
Gas-bubble formation occurs at the solidification front
(solid/liquid interface) where the gaseous elements are
rejected from the solid and concentrate in the liquid (in
the same way that segregations occur). Blowholes and
pinholes can not only effect the internal slab quality, but
also cause surface defects. The primary gases contri-
buting to blowhole formation in steel are CO, H
2
and
N
2
.
25
p(H
2
)+p(N
2
)+p(CO) > p(atm) + p(Fe) +
2 r
(6)
J. BURJA et al.: NITROGEN AND NITRIDE NON-METALLIC INCLUSIONS IN STEEL
Materiali in tehnologije / Materials and technology 53 (2019) 6, 919–928 921
Figure 2: Solubility of nitrogen in pure iron (or low-alloy steels) at 1
atm pressure of N
2
24
Figure 1: Calculated influence of alloying elements on nitrogen
solubility using interaction coefficients given in Table 2, p = 1 atm,
T = 1873 K (1600 °C)
Figure 3: Example of a small gas bubble in a continuously cast billet
(30MnVS6), as-cast condition, etched with nital 4 %

where p(atm) is the atmospheric surface pressure on the
liquid in the mould, p(Fe) is the ferro-static pressure at
the location of the blowhole, and is expressed as:
p(Fe) = Fe
·g·h, (7)
(the density of liquid iron Fe
= 7 g/cm
3
,
26
g = 9.81 m/s
2
,
h is the height in meters).
 is the surface tension of the liquid steel in contact
with the gas bubble of radius r. The surface tension of
liquid iron at 1873 K is 1890 mN/m
27
.
Pinholes and blowholes form when the following
conditions are met:
p(H
2
)+p(N
2
)+p(CO) > 1.5 atm
25
(8)
In such cases, proper vacuum degassing or the addi-
tion of nitride-forming elements can prevent the forma-
tion of a defect like that shown in Figure 3.
3.2 Solid solution
The effect of nitrogen on the mechanical properties is
the result of interstitial solid-solution strengthening by
the free nitrogen. A high nitrogen content may result in
inconsistent mechanical properties in hot-rolled pro-
ducts, embrittlement of the heat-affected zone (HAZ) of
welded steels, and poor cold formability. Interstitial
solid-solution nitrogen can result in strain ageing that is
caused by the interaction of the interstitial atoms and the
elastic stresses related to the presence of dislocations.
28
High nitrogen in vanadium-based micro-alloyed steels
was considered to cause unsatisfactory HAZ by solute
nitrogen toughness degradation. But R. Lagneborg et
al.
29
showed that this is not true because the HAZ
toughness strongly depends on the welding parameters,
mainly the heat input.
4 NITRIDES
In most steel compositions nitrides do not form in the
liquid steel if the solubility products are taken into
account. With proper steelmaking and casting practices
these compounds most often precipitate during the
cooling of the solidified steel along the austenite grain
boundaries. These precipitates influence the mechanical
properties of the steel both at room temperature and hot
working temperatures. Nitride precipitates can have a
beneficial effect on the steel properties, from grain
refining to precipitation hardening. However, when
nitrides are formed in the melt or during solidification
they may be considered as non-metallic inclusions and
do not have the same beneficial effects. Typically, Al,
Nb, Ti, V, Zr and also B form nitride inclusions in steel.
30
Nitrides are known to inhibit grain growth, if the critical
grain radius has not been reached, be it during high-tem-
perature annealing, slab soaking, or during welding in
the heat-affected zone.
31,32
Nitrides can also control the
grain size evolution during hot working (rolling, forg-
ing). Nevertheless, micro-alloying additions that interact
with nitrogen have a direct influence on the recrystalli-
zation behaviour. The thermodynamic conditions for the
formation of such coarse nitride inclusions are described
and examples from experiments and industrial practice
are shown. The coarse nitrides cause the loss of the aus-
tenite grain-boundary pinning effect that is provided by
the micro-alloying elements during hot deformation
(rolling, forging).
In some cases of alloyed steels, nitrides can precipi-
tate in the liquid steel (above liquidus temperature, T
liq
)
or during solidification (between liquidus and solidus,
T
sol
temperature of particular steel grade). These coarse
nitrides usually have a detrimental effect on the steel
properties and are considered as non-metallic inclusions.
The promotion of a quasi-cleavage brittle fracture and
decreased impact toughness can occur due to coarse
nitride non-metallic inclusions, as shown by an example
of a titanium nitride (TiN) in Figure 4.
In the case of coarse niobium carbonitrides a drastic
decrease of deformation properties perpendicular to the
J. BURJA et al.: NITROGEN AND NITRIDE NON-METALLIC INCLUSIONS IN STEEL
922 Materiali in tehnologije / Materials and technology 53 (2019) 6, 919–928
Figure 5: Coarse carbonitride cracking during hot working (0.17 w/%
C, 0.43 w/% Nb, 0.060 w/% N)
Figure 4: Quasi-cleavage brittle fracture caused by the presence of
coarse nitrides (0.14 w/% C, 0.02 w/% Nb, 0.0058 w/% N)

surface of the hot rolled products is expected, tested in
accordance with the standard SIST EN 10164:2018.
33
The reaction equilibrium for the formation of metal
nitride in liquid iron can be written as:
(MN) = [M] + [N] (9)
where M represents a nitride forming metal.
The values of the Gibbs free energies for nitride for-
mation in Equations (11, 13, 15, 17 and 19) were taken
from HSC Chemistry 8.0.1 and data from Sigworth and
Elliot was used for the transformation from mole to
weight percent with the infinitely dilute iron solution.
34
4.1 Aluminium nitrides
Usually, aluminium nitrides (AlN) secondary phase
particles precipitate on the grain boundaries as fine
particles in Al killed steels. These precipitates supress
grain growth during heat treatment.
35–37
However, AlN as
inclusions occurs in high-Al steel. In the case of other
micro-alloying elements the solubility product of AlN is
drastically influenced. The AlN inclusions can preci-
pitate due to the enrichment of the solute in liquid phase
during solidification and induce intergranular fracture.
38
Hard AlN inclusions are also crack-initiation sites and
can decrease hot ductility.
39,40
Figure 6 shows an example
of an aluminium nitride inclusion from an industrial
sample of NAK80 age-hardening plastic mould steel. In
this particular case the aluminium nitrides proved to be
problematic during the finish polishing of the mould.
Tools for plastic moulding require a high surface finish,
especially for transparent or high-gloss products. The
concentrations of aluminium and nitrogen needed for
AlN inclusion formation at 1873 K (1600 °C) calculated
using Equation (11) are shown in Figure 7.
[Al] + [N] = AlN (10)
 G
0
= –269269.96 J +121.2 J/K·1873 K (11)
4.2 Boron nitride
Soluble boron is normally used to improve the steel
hardenability, which is directly related to the mechanical
properties achieved in the final state. However, BN pre-
cipitates or inclusions do not contribute to the steel
hardenability and do not have a significant role in the
martensitic transformation, in fact BN are usually con-
sidered as boron loss. Interestingly, BN inclusions have
been known to improve machinability; they have been
J. BURJA et al.: NITROGEN AND NITRIDE NON-METALLIC INCLUSIONS IN STEEL
Materiali in tehnologije / Materials and technology 53 (2019) 6, 919–928 923
Figure 7: AlN equilibrium precipitation in Fe(l) at 1873 K for
different Al/N concentrations
Figure 6: Mapping of a coarse AlN non-metallic inclusion (1 w/% Al, 0.008 w/% N)
Figure 8: BN nucleated on an alumina inclusion (0.1 w/%B, 0.02 w/%
N)

successfully used in the development of free machining
stainless steels. BN inclusions can form during the steel
solidification process or as secondary phases, they are
usually small, so unlike MnS, they do not worsen the
mechanical properties of steel.
41–47
Figure 8 shows an
example of a boron nitride inclusion from an experi-
mental laboratory steel sample with 0.1 w/% B and 0.02
w/%N. The concentrations of boron and nitrogen needed
for BN inclusion formation at 1873 K (1600 °C)
calculated using Equation (13) are shown in Figure 9.
[B] + [N] = BN (12)
 G
0
= –190093.5 J +84.0 J/K·1873K (13)
4.3 Niobium nitrides
Niobium can be, like titanium, used to stabilise stain-
less steels.
48,49
Niobium has a smaller tendency to form
large detrimental nitrides than titanium. In fact, niobium
is more prone to form carbides. Niobium carbo-nitride
precipitates are known to decrease hot ductility.
40,50,51
Niobium nitrides and carbo-nitrides delay the onset of
dynamic recrystallization during hot working, inhibit
austenite grain growth during high-temperature anneal-
ing and therefore play an important role in grain refine-
ment.
37,52
Figure 10 shows an example of a niobium
nitride inclusion from an industrial sample of
X19CrMoNbVN11-1 creep-resistant steel. The sample in
Figure 10 was taken after hot forging; void formation is
visible at the nitride corners along the direction of
material flow. The concentrations of niobium and
nitrogen needed for NbN inclusion formation at 1873 K
(1600 °C) calculated using Equation (5) are shown in
Figure 11.
[Nb] + [N] = NbN (14)
 G
0
= –221557.9 J +102.1 J/K·1873K (15)
4.4 Titanium nitrides
Titanium is often added to steels for grain size con-
trol. This is an important role in HSLA (high strength
low alloy) steels, where it also effects the critical
deformation needed for the dynamic recrystallization
during hot rolling. Some special applications include the
stabilization of stainless steels, where it prevents the
formation of chromium carbides and intergranular corro-
sion.
48,49
On the other hand, titanium nitrides can prom-
ote local corrosion in stainless steels.
49,53
TiN formation
in ferritic stainless steels refines the microstructure as it
promotes the heterogeneous nucleation of delta
ferrite.
23,54
The addition of titanium can also prevent BN
formation to improve the effect of boron on steel harden-
ability for HSLA and similar grade steels.
55
The form-
ation of solid TiN inclusions in the melt can also lead to
nozzle clogging or even "floater formation" during
continuous casting.
56,57
The TiN particles themselves
preferably nucleate on Mg-Al oxides, where the Mg
content increases the TiN nucleation potential.
54,57–59
J. BURJA et al.: NITROGEN AND NITRIDE NON-METALLIC INCLUSIONS IN STEEL
924 Materiali in tehnologije / Materials and technology 53 (2019) 6, 919–928
Figure 9: BN equilibrium precipitation in Fe(l) at 1873 K for different
B/N concentrations
Figure 11: NbN equilibrium precipitation in Fe(l) at 1873 K for
different Nb/N concentrations
Figure 10: Coarse niobium nitrides (0.2 w/% C, 0.5 w/% Nb, 0.075
w/% N)

These inclusions can decrease ductility and cause crack
initiation and propagation; their sharp edges are
especially harmful for stress concentrating.
2
Figure 12
shows an example of a titanium nitride inclusion from an
industrial sample of H13 hot-work tool steel. The
concentrations of titanium and nitrogen needed for TiN
inclusion formation at 1873 K (1600 °C) calculated
using Equation (17) are shown in Figure 13.
[Ti] + [N] = TiN (16)
 G
0
= –293703.2 J +106.4 J/K·1873 K (17)
4.5 Zirconium nitrides
In the steel melt zirconium has a high affinity to both
oxygen and nitrogen.
10,60,61
In a fully de-oxidized steel,
zirconium combines with nitrogen in preference to
sulphur or carbon and forms ZrN nitrides.
62
Because ZrN
are already present in the melt they can also serve as
nucleation sites for MnS.
61
The addition of Zr through
the formation of ZrN can have a significant effect on the
increased ductility and toughness by refining the
austenite grain size.
62
Figure 14 shows an example of a
zirconium nitride inclusion from an experimental labo-
J. BURJA et al.: NITROGEN AND NITRIDE NON-METALLIC INCLUSIONS IN STEEL
Materiali in tehnologije / Materials and technology 53 (2019) 6, 919–928 925
Figure 14: Coarse zirconium nitride with a zirconium oxide nucleus (0.11 w/% C, 0.015 w/% Zr and 0.01 w/%N)
Figure 12: Coarse titanium nitrides (0.38 w/% C, 0.005 w/% Ti, 0.007
w/% N)
Figure 13: TiN equilibrium precipitation in Fe(l) at 1873 K for
different Ti/N concentrations

ratory steel sample. The nitride nucleated on a zirconium
oxide inclusion. The concentrations of zirconium and
nitrogen needed for ZrN inclusion formation at 1873 K
(1600 °C) calculated using Equation (17) are shown in
Figure 15.
[Zr] + [N] = ZrN (18)
 G
0
= –312121.2 J +116.8 J/K·1873K (19)
5 CONCLUSIONS
Generally speaking, nitrogen represents an impurity
in steel, but when strong nitride-forming elements are
alloyed to the steel in small amounts (micro-alloying) it
takes on an important role.
Non-metallic inclusions obviously have a negative
effect on mechanical properties, especially on ductility
(toughness, formability). This is also the case with
coarse nitrides that form in the steel melt. Coarse nitrides
are even more problematic, because they deplete the
matrix of micro-alloying elements. This negatively
effects the formation of fine nitride precipitates that
positively contribute to target strength and toughness.
Both properties are strongly related to grain size: the
finer the grains the higher the strength and toughness.
The effect of fine nitride precipitation can be utilised to
achieve a fine-grained microstructure in products during
hot working and/or final heat treatment.
The titanium nitride is the most representative nitride
non-metallic inclusion, as it is most commonly found in
steels. The nitride non-metallic inclusions also have a
negative effect on the steel properties, because of their
relatively large size (above 5μm) and sharp-edged shape.
They act as stress risers. This is why it is crucial that the
excessive precipitation of coarse nitrides in the steel melt
should be prevented or minimized.
Thermodynamic calculations can help us both predict
the conditions for nitride formation as well as the equi-
librium solubility of nitrogen under different nitrogen
gas partial pressures, temperatures and melt composi-
tions.
Acknowledgements
This research was made as a part of ^MRLJ research
project co-financed by the Republic of Slovenia and the
European Union under the European Regional Develop-
ment Fund.
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