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NITROGEN ALLOYING OF HIGH CHROMIUM STEELS BY GAS INJECTION IN
THE LADLE
Pavel Machovčák1)*, Zdeněk Carbol1), Aleš Opler1), Antonín Trefil1), Jiří Bažan2), Ladislav
Socha2)
1)
VÍTKOVICE HEAVY MACHINERY a.s., Ostrava, Czech Republic
2)
VŠB – Technical University of Ostrava, Faculty of Metallurgy and Material Engineering,
Ostrava, Czech Republic
Received: 10.12.2013
Accepted: 18.04.2014
*Corresponding author: e-mail: [email protected], Tel.: +420 595 954 394,
VÍTKOVICE HEAVY MACHINERY a.s., Ruská 2887/101, Vítkovice, 703 00 Ostrava, Czech
Republic
Abstract
There is a steadily increasing demand for steel grades with increased nitrogen content in a
certain range. Nitrogen is an important alloying element and it has a beneficial effect on
mechanical and corrosion properties of stainless steel. Increased nitrogen content has a positive
effect on the increasing of the pitting resistance equivalent in case of duplex stainless steels.
Nitrogen can also substitute expensive nickel that strongly supports its use. There are several
ways to increase nitrogen content in the steel. One of the most used ways is the nitrogen gas
injection in the ladle. Solubility of nitrogen is significantly influenced by the chemical
composition of the steel. However, due to nitrogen enrichment in the remaining liquid, gas pores
can be formed during solidification of nitrogen-rich stainless steels. This article describes the
results obtained during production of high nitrogen steels at steel plant VÍTKOVICE HEAVY
MACHINERY a.s., mainly during production of martensitic 9% chromium steels and super
duplex stainless steels.
Keywords: Nitrogen, Nitrogen solubility, High Nitrogen Steel, Martensitic steel, Super Duplex
Stainless Steel
1 Introduction
Nitrogen is an important element influencing the properties of high alloyed steels, although its
content is much lower than the content of the other alloying elements. For example, in the case
of 9-12 % chromium steels higher nitrogen content supports the precipitation of particles of
vanadium nitride, VN, which leads to increase the creep resistance of these steels with
increasing nitrogen content [1]. In the case of two-phase, austenitic-ferritic steel, nitrogen affects
corrosion resistance of these steels, mechanical properties and has significant influence on the
phase composition, i.e. the ratio of austenite and ferrite. Most often the ferrite content in these
steels is between 40 and 60 percent [2]. An increase of nitrogen in the duplex microstructure has
several significant effects on the phase diagram. Nitrogen additions have a strong stabilization
effect on austenite when considering the high temperature ferritic transformation [3], [4].
Considering the fact that nitrogen is element, which is able to stabilized austenite, it can be used
as an inexpensive substitute for other more expensive elements. Nitrogen is the cheapest
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alloying element – 1 kg of nitrogen replaces 6 to 20 kg of nickel [5], [6]. Several different ways
of nitrogen alloying can be used in metallurgical practice. There are two basic mechanism by
which nitrogen is transferred to the melt. The first is nitrogen pick-up via reaction at the gasmelt interface, where diatomic N2 gas molecule dissociate and are absorbed into melt as atomic
nitrogen. The second method is the introduction of nitrided compounds directly into liquid
metals. The most common examples of high-nitrogen additives are nitrided ferrochrome
(FeCrN), ferromanganese (FeMnN), ferrovanadium (FeVN) or nitrided lime Calzot® (CaCN2).
Both methods can be applied both at atmospheric as well as at elevated pressure [7], [8], [9].
2 High chromium steels alloyed with nitrogen
There are a number of steel alloyed with nitrogen. High nitrogen stainless steel are becoming an
increasingly important new class of engineering materials. High chromium steels alloyed with
nitrogen are commonly used for example in the power and petrochemical industry. The
requirements put on these steels for conventional and nuclear power engineering include in
particular achieving of strength properties at increased temperatures, a sufficient level of fracture
toughness, good weldability, sufficient corrosion resistance, structural stability and resistance to
radiation damage. Materials used for the petrochemical industry must resist not only corrosive
impact of oil, but also must resist the influence of salty seawater. Further, these steels must have
a higher strength, high resistance to intergranular corrosion and reduce tendency to stress
corrosion cracking. For these reasons, it is very common using of modified steels containing 912 wt. % chromium, for example P91/T91, P92/T92, X20CrMoV12-1, 12CrMoVNbW (COST
E), which are used for production of steam pipes, parts of steam turbines as well as austenitic
stainless steels, for example 316LN, 347. Duplex stainless steels are very suitable for
applications in petrochemical industry. Increased resistance to pitting corrosion can be achieved
by higher content of alloying elements. The positive effect of nitrogen has led to modified
empirical PRE (Pitting Resistance Equivalent) by a nitrogen factor.
PREN = %Cr + 3.3 x %Mo + (16-30) %N
(1.)
Using the PRE number can be distinguished between duplex, super-duplex and hyper-duplex
stainless steels. In the case of duplex stainless steels the value of PRE is from 35 to 40, in the
case of the super-duplex stainless steels the value of PRE must be higher than 40. When value of
PRE is higher than 48 it is already hyper-duplex stainless steel [3], [6].
2.1 Nitrogen solubility in high alloyed steels
The solubility of nitrogen in pure iron is only 0.0450 wt. % at 1600 °C and at atmospheric
pressure (101 325 Pa) [10]. Nitrogen solubility in liquid Fe-based alloys generally follows
Sievert´s law and is proportional to the square root of N2 gas pressure over the melt [7], [11].
Nitrogen-alloying offers unique challenges since the solubility of nitrogen in liquid-Fe and Febased alloys are limited at atmospheric pressure. However, nitrogen solubility can be increased
by increasing the nitrogen gas pressure above melt or through alloying additions. Steel should be
considered “high-nitrogen” if it contains more nitrogen than can be retained in the material by
processing at atmospheric pressure. For ferritic materials, this limit is about 0.08 wt. % N, while
for most austenitic materials it is about 0.4 wt. % [7]. Solubility of nitrogen is therefore very
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strongly affected by alloying elements. Additions of Cr and Mn increase, and Ni reduces the
nitrogen solubility. Therefore, the solubility of nitrogen in Fe-Cr-Ni is lower than in Fe-Cr-Mn
alloys with comparable Cr concentrations. Some of elements which have the greatest positive
influence on nitrogen solubility in the liquid, such as Ti, Zr, V and Nb, also have a strong
tendency to form nitrides. Chromium is not only the most important alloying element in stainless
steels, it also significantly increases nitrogen solubility in liquid iron alloys with a lesser
tendency, compared to Ti, Zr, V, Nb for nitride formation in the solid-state. Although Ni is an
important alloying element in stainless steels, its negative influence on nitrogen solubility has
led to reductions in its levels in most high-nitrogen alloys. Manganese is used extensively in
many high-nitrogen stainless steels to increase nitrogen solubility.
Although the surface active elements oxygen, sulfur, selenium and tellurium only slightly affect
nitrogen solubility in pure iron, they may change the rate of nitrogen transfer by orders [12].
The optimum content of nitrogen that may be present within the range of its solubility in high
alloy steel at normal pressure for improvement of their properties can be calculated from the
following equation:
%N =
%
%
−
%
(2.)
It is known from practice that up to 0.7 wt. % of nitrogen (depending on the alloy system) can
dissolve during steelmaking and casting processes in open units [13]:
• 0.25 – 0.35 wt. % in Cr-Ni steels of type 18-10
• 0.35 – 0.45 wt. % in steels Fe-Cr-Ni-Mn alloy system
• up to 0.7 wt. % in steels of Fe-Cr-Mn alloy system
Nitrogen content more than 1 wt. % can be achieved in high-alloy steel at pressure higher than 4
MPa [14].
2.2 Gas pores in nitrogen alloying stainless steels
However, due to nitrogen enrichment in the remaining liquid, gas pores can be formed during
solidification of nitrogen-rich stainless steels. The formation of nitrogen gas pores is closely
related to the solubility of nitrogen. The amount of nitrogen gas pores increased with initial
nitrogen contents of castings. Manganese is an element that stabilizes the austenite phase and
increases nitrogen solubility at the same time, since nitrogen has a much higher solubility in
austenite than ferrite phase. So it can be expected that an increase of Mn content in duplex
stainless steel can suppress the formation of nitrogen gas pores [15], [16].
3
Verification of the influence of sulfur content on the nitrogen transfer under the
laboratory conditions
Laboratory experiments focused on nitrogen alloying of liquid steel by gas injection were
carried out in laboratories of the Department of Metallurgy and Foundry, VŠB-Technical
University of Ostrava. The influence of sulfur and chromium content on nitrogen alloying
efficiency was verified. Experiments were carried out using induction furnace equipped with
high frequency generator GV22. Weight of base metal charge was 300 grams. The metal charge
was placed in a magnetite crucible which was situated in a graphite block, which is located in
the electromagnetic field of the inductor, see Fig. 1.
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155
A schematic diagram of experimental system [8] 1 – protective tube from SiO2; 2 water cooled inductor; 3 – graphite block; 4 – magnesite crucible; 5 – protective
powder form Al2O3; 6 – melted steel; 7 – cover; 8 – heat resistant cover; 9 – argon
pipe; 10 – seal; 11 – cover; 12 – fireclay brick; 13 – thermocouple PtRh6-PtRh30; 14
– nozzle from Al2O3; 15 – connecting tube; 16 – rotametters; 17 – reducing valve
Nitrogen was blown through alumina tube from the top of the bath. Argon was blown into
crucible during melting because of limitation of the oxidation of the metal charge. After melting
of the charge and temperature stabilization, ferosulfur was added into liquid steel to achieve
required different levels of sulfur. The sulfur content was targeted to content about 0.020 wt. %,
0.050 wt. %, 0.100 wt. %, 0.150 wt. %. Nitrogen blowing on the metal surface was started at
temperature of 1600 °C. This temperature was chosen because of the approximation to the
normal operating conditions for nitrogen gas alloying at steel plant. Nitrogen blowing time was
30 minutes during all experiments. Samples of steel were taken after 5, 10, 20 and 30 minutes of
blowing. Two different steel grades were used for laboratory experiments. Chemical
compositions of both steel grades are shown in Table 1. There is a big difference in the
chromium content of these two steel grades.
Table 1 Chemical composition of two experimental steel grades
Chemical composition (wt. %)
Steel
C
Mn
Si
P
S
Cu
Ni
Cr
grade
Low Cr
0.34 0.66 0.23 0.004 0.003 0.14 0.84 1.05
content
High Cr
0.74 0.37 0.15 0.010 0.012 1.78 5.46 25.81
content
Mo
Al
N
0.23
0.035
0.0116
3.31
0.009
0.0580
3.1 Results and discussion
The measured nitrogen content during nitrogen blowing depending on the initial sulfur content
in low chromium steel are shown in Table 2. Each value of the nitrogen content was determined
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as the average of the values from the three measurements. The following finding resulting from
the performed measurement with the samples from low chromium steels (1.05 wt. % Cr) could
be presented:
• Sulfur content in low chromium steel negatively affect the dissolution of nitrogen in
steel
• When the sulfur content is higher than 0.070 wt. % steel surface is probably covered
with surface active sulfur, which negatively affects the dissolution of nitrogen
• Whereas the initial nitrogen content is 0.0116 wt. % it can be concluded that the
effectiveness of the nitrogen alloying is the best in the first five minutes of nitrogen
blowing
• The maximum nitrogen content, which was achieved, was 473 ppm
Table 2 The nitrogen content in low chromium steel samples at different time of nitrogen
blowing and with varying sulfur content
Sulfur content before start of nitrogen blowing (wt. %)
Time of
0.016
0.029
0.074
0.096
0.179
nitrogen
blowing (min)
Nitrogen content during nitrogen blowing (wt. %)
5
0.0389
0.0237
0.0224
0.0279
0.0341
10
0.0416
0.0385
0.0350
0.0329
0.0322
15
0.0436
0.0368
xxx
0.0345
xxx
20
0.0473
0.0415
0.0401
0.0384
0.0404
The measured nitrogen content during nitrogen blowing depending on the initial sulfur content
in high chromium steel are shown in Table 3. The following finding resulting from the
performed measurement with the samples from low chromium steels (25.81 wt. % Cr) could be
presented:
• Effectiveness of the nitrogen alloying is the highest during the first five minutes –
similarly as in the case of steel with low chromium content
• Much higher nitrogen content was achieved in comparison to steel with low chromium
content. Maximum achieved nitrogen content was 5410 ppm, it is thus 11 times more
than in case of low chromium steel
• The influence of surface active sulfur has not been demonstrated in this case of steel
with very high chromium content. Nitrogen content was approximately the same in
samples with very high sulfur content as well as in samples with very low sulfur
content.
• Positive effect of high chromium content in the steel which significantly affects the
dissolution of nitrogen prevails over the negative effect of surface active sulfur.
4
Nitrogen alloying of high chromium steels by gas injection in the ladle under conditions
of steel plant VÍTKOVICE HEAVY MACHINERY a.s.
High chromium steels or generally stainless steel are produced mainly at VOD or AOD
facilities. High chromium steel melt with high carbon content is processed on both of these
aggregates. The main purpose of these treatments is to reduce carbon content under vacuum
condition or under argon atmosphere. Therefore, the nitrogen is alloyed at the end of processing
after finishing of vacuum processing [17], [18], [19].
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Table 3 The nitrogen content in high chromium steel samples at different time of nitrogen
blowing and with varying sulfur content
Sulfur content before start of nitrogen blowing (wt. %)
Time of
0.012
0.021
0.042
0.108
0.118
nitrogen
blowing (min)
Nitrogen content during nitrogen blowing (wt. %)
0
xxx
0.0914
0.2000
0.1670
0.1200
5
xxx
0.0993
0.2370
0.2870
0.2220
10
xxx
0.1440
0.3050
0.3810
0.3130
20
xxx
0.2460
0.4480
0.4990
0.4620
30
xxx
0.2810
0.4870
0.5410
0.5270
High chromium steels with increased nitrogen content are produced in VÍTKOVICE HEAVY
MACHINERY a.s. (further also VHM) for several years. Particularly, steel grade P91 according
to ASTM A335 (or X10CrMoVNb9-1 according to EN), with the desired nitrogen content in the
range 300 – 700 ppm (0.0300 – 0.0700 wt. %) is here produced long term and regularly. Next
examples of high alloyed steels with increased nitrogen content produced in VHM are:
X4CrNiMo16-5-1, which requirement for nitrogen content is in the range of 300 and 500 ppm.
Also super duplex stainless steel UNS S32550 (equivalently W.Nr. 1.4507 or X2CrNiMoCu256-3 according to EN) was produced here in the framework of research project [20]. The required
nitrogen content of this steel grade is in the range of 1000 – 2500 ppm. To meet the requirement
to achieve a value of pitting resistance equivalent 40 at minimum (according to equation (1.)) it
is necessary to achieve the nitrogen content in range of 2000 – 2500 ppm.
Production technology route EAF – LF –VOD is mainly preferred for these high chromium
steels with increased nitrogen content. Only in exceptional circumstances the technological route
EAF – LF – VD is used [21]. Nitrogen alloying of steel is performed only by gas injection in the
ladle. All secondary metallurgy facilities (LF, VD, VOD) are equipped with nitrogen supply.
Gas flow is up to 1000 liters per minute. Nitrogen flow and its total amount are recorded in the
information system of steel plant. Nitrogen is always alloyed into steel at the end of treatment,
i.e. after carbon oxidation at VOD, eventually after vacuum treatment at VD. Sufficient thermal
reserve must be created for nitrogen alloying. In case of steel grade P91, nitrogen content in steel
is in the range of 150 and 350 ppm before the oxidation of carbon at VOD. This content depends
mainly on composition of the metal charge and on the way of melting at EAF. Nitrogen content
in steel is in the upper part of this range, i.e. 300 – 350 ppm, in case that the melting of alloyed
scrap in reduction condition is used. If traditional melting of unalloyed scrap with intensified
usage of oxygen is used and chromium is alloyed during tapping and during processing of steel
at ladle furnace, nitrogen content is at the bottom of this range, i.e. 150 – 200 ppm. Oxidation of
carbon at VOD is accompanied by the significant reduction of nitrogen content in steel. After
finishing of this process, nitrogen content is in the range of 50 and 100 ppm. To meet the
required nitrogen content is therefore necessary to increase its content of about 400 ppm.
Considering the weight of heat 45 tons, it is required to alloy 18 kg of nitrogen into steel, which
is 14.4 Nm3 under normal conditions. Based on the evaluation of performed heats, average rate
of nitrogen pickup is 9.6 ppm per minute at nitrogen flow 1000 liters per minute. The highest
nitrogen pickup rate is at the beginning of nitrogen alloying, almost 15 ppm˖min-1, whereas there
is decline of nitrogen pickup rate during nitrogen blowing up to 4.9 ppm˖min-1. Temperature
drop during nitrogen alloying was in the range from 0.5 to 0.9 C˖min-1 and depends on the
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condition of the used ladle. Average utilization of blown nitrogen was 61.6 %. The final content
of nitrogen in steel grade P91 produced in VHM is in the range from 440 to 670 ppm and
nitrogen alloying time is from 40 to 65 minutes. Process of nitrogen alloying at the range from
400 to 700 ppm can be handled without additional heating of liquid steel.
In case of production of high alloyed super duplex stainless steel UNS S32550, which contains
26 wt. % Cr, production technology of melting of alloyed scrap in reduction conditions without
usage of oxygen was used. Nitrogen content in the steel in this stage of steel making production
was significantly higher than in the case of the steel grade P91, it was almost 500 ppm. Nitrogen
content has increased during tapping and LF treatment up to 730 ppm. Subsequently it decreased
to level 425 ppm after carbon oxidation at VOD. It is necessary to increase nitrogen content at
least on 2000 ppm due to the requirement to the value of PRE at least 40 and also due to achieve
the desired phase composition. Only nitrogen alloying by gas was used for this experimental
heat. Twenty tons ladle was used for this trial because of very high price of this steel grade.
Required amount of nitrogen to increase its content about 2000 ppm in case of 18 tons of liquid
steel was 36 kg, i.e. 28.8 Nm3. Average nitrogen pickup rate was 25 ppm˖min-1 while nitrogen
flow was 1000 liters per minute. Final nitrogen content in super duplex stainless steel was 2450
ppm, i.e. 0.2450 wt. %. Nitrogen alloying time was 81 minutes. Utilization of nitrogen was 89 %
[8].
5 Conclusions
In this work, the nitrogen alloying of high chromium steels by gas injection in the ladle was
studied. Laboratory experiments were focused on verification of the influence of sulfur content
on the nitrogen transfer into steel. It has been shown that sulfur content in low chromium steel
negatively affects the dissolution of nitrogen in steel. Solubility of nitrogen is much higher in
high chromium steels in comparison to steel with low chromium content. Positive effect of high
chromium content in the steel which significantly affects the dissolution of nitrogen prevails
over the negative effect of surface active sulfur. Nitrogen alloying of high chromium steels by
gas in the ladle is used at steel plant VÍTKOVICE HEAVY MACHINERY a.s. It is possible to
achieve very high nitrogen content using this method. In the case of super duplex stainless steel
can be achieved more than 2400 ppm of nitrogen under atmospheric conditions. The utilization
of nitrogen gas is in the range from 60 to 90 percent and highly depends on the chemical
composition of steel.
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Acknowledgements
Authors are grateful for the support of experimental works by project FR-TI1/351 under
financial support of Ministry of Industry and Trade of Czech Republic.
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