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\begin{document}

\title{Influence of Microalloying Elements (Ti, Nb) and Nitrogen Concentrations
on Precipitation of Linepipe Steels - A Thermodynamic Approach}



\author[1]{Sima Aminorroaya Yamini}%
\affil[1]{Sheffield Hallam University Faculty of Arts Computing Engineering and Sciences}%


\vspace{-1em}



  
  \date{June 12, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
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\selectlanguage{english}
\begin{abstract}
A CALculation of PHase Diagrams (CALPHAD) approach was used to study the
formation temperatures of nitride and carbonitride precipitates in
nominated linepipe steels. The calculated results were in good agreement
with relevant experimental data reported in the literatures, where the
optimum titanium to nitrogen ratio and austenite grain growth was
studied in similar steel compositions. The niobium concentration up to
0.12 wt\% showed no influence on the formation temperature of nitrides
in a common linepipe steel composition, while significantly increased
the precipitation temperature of niobium carbides up to 0.03 wt\%.
Nitride precipitates contained a high concentration of titanium while
niobium contributed mostly to the formation of carbide/carbonitrides.
Although the dissolution and growth of precipitates are controlled
kinetically, the thermodynamic calculation approach can be used to
efficiently predict the equilibrium amount and composition of the stable
phases in chemically complex systems. This results in a more accurate
design of experiments, to minimise the number of tests required to
obtain optimum chemical compositions and heat treatment procedures.%
\end{abstract}%



\sloppy


\section*{Introduction}

{\label{introduction}}

Nitride {[}1, 2{]}, carbide {[}3{]} and carbonitride {[}2{]}
precipitates in steels pin grain boundaries. Depending on the chemical
composition of the steel, above certain temperatures during heat
treatment, rolling and/or welding, the austenite grains tend to coarsen
because the precipitates are taken into solution, leaving the grain
boundaries unimpeded {[}4-6{]}. Formation and coarsening of nitride and
carbonitride precipitates, particularly in microalloyed steels, have
been extensively studied experimentally due to their influence on the
mechanical properties of products {[}1{]} and the formation of cracks
during shaping of steels {[}7, 8{]}.

Titanium Nitride (TiN) precipitates in microalloyed steels extend the
temperature range over which the grain boundary pinning is effective,
due to a significantly higher solution temperature of these precipitates
compared to other precipitates that are known to restrict grain growth
{[}1,2{]}. Therefore, extensive efforts have been made to optimise the
Ti/N ratio as a major microstructural control factor for a variety of
steels {[}1, 2, 9-11{]}. A high concentration of Ti and N promotes the
formation of coarse TiN precipitates that deteriorate toughness.
Consequently, a reduced content of Ti and N is suggested while
simultaneously retaining a minimum Ti/N ratio to obtain suitable
hardenability {[}1{]}.

It should be recognised that the dissolution and growth of TiN
precipitates and austenite grains are kinetically controlled phenomena.
Therefore, time is a governing factor for the size and distribution of
precipitates. However, the formation temperatures of precipitates can be
determined thermodynamically as a function of chemical compositions.
This information guides the design of experiments and consequently
minimises the experimental data required to optimise the Ti/N ratio for
various compositions. In the current study, the effect of Ti, Niobium
(Nb) and N concentrations are assessed on precipitation of common
linepipe steels as a case study using CALPHAD (CALculation of PHAse
Diagrams) method. The approach presented here can be applied to various
steel compositions for more efficient design of experiments.

METHOD

Several industrial compositions of linepipe steels were adopted to
calculate the phase diagrams and study the composition and formation
temperature of nitride and carbonitride precipitates, aiming to explore
the possibility of calculated methods to minimise the number of
experiments required to develop the optimum Ti/N ratio and microalloying
element concentrations. The ThermoCalc software package {[}12{]}, using
the thermodynamic databases of TCFE7 {[}13{]}, was employed to study the
composition and formation temperature of nitride and carbonitride
precipitates in titanium and niobium microalloyed linepipe steels. This
software package is based on minimizing the Gibbs free energy of the
individual phases in the equilibrium state. It uses the CALPHAD method
to extrapolate thermodynamic descriptions for use in an n-component
system, based on the assessment of binary and available ternary and
quaternary experimental data stored in the thermodynamic database. The
thermodynamics of the liquid phase is described by a regular solution
model and the solid phases by sub-lattice models. The phase equilibria
are calculated by a free-energy minimization determined by a
Newton-Raphson technique. The databases are used to calculate the phase
fractions, phase compositions and transformation temperatures under
thermodynamic equilibrium conditions {[}14, 15{]}, providing an
efficient calculation technique to predict the equilibrium amount and
composition of the stable phases of complex system under a given set of
conditions {[}16{]}. Phase diagram sections can be determined with up to
five independent variables in a very complex multi-component and
heterogeneous system (up to 40 elements and 1000 species) {[}13{]}.

RESULTS AND DISCUSSION

\textbf{Table 1} summarises the chemical composition of a number of
common linepipe steels with various Ti/N ratios. The linepipe steels
contain elements such as phosphorous, sulphur, calcium and aluminium in
trace levels, which have marginal impact on the composition and
temperatures of the precipitates within agreed compositions. These
elements were disregarded in the calculation of phase diagrams to
minimise the complexity of calculations and improve the feasibility of
achieving reliable results from the calculated phase diagrams. All
calculations were performed above 900 K where the diffusion of the
interstitial elements in the steel matrix is fast enough to reduce the
kinetic effect on nucleation and the chemical composition of
precipitates. For the Nb and Ti microalloying elements to diffuse one
micrometre at 900\selectlanguage{ngerman}°C (1173K), it takes approximately 12 minutes and 14
seconds respectively, according to the diffusion coefficients for Nb
{[}17{]} and Ti {[}18{]} in steel. Therefore, the calculated phase
diagram should be treated with extra care at lower temperatures.

Two sets of calculations were performed, based on fixed Ti
concentrations and varying N contents. The compositions are referred to
as Low-Ti and High-Ti, according to the Ti concentration in the steels
indicated in Table 2.

\textbf{Table 1} Chemical composition of common linepipe steels (wt\%).\selectlanguage{english}
\begin{longtable}[]{@{}lllllllllll@{}}
\toprule
\textbf{C} & \textbf{Mn} & \textbf{Si} & \textbf{Nb} & \textbf{Ti} &
\textbf{V} & \textbf{Ni} & \textbf{Cr} & \textbf{Mo} & \textbf{N} &
\textbf{Ti/N}\tabularnewline
\midrule
\endhead
0.05 & 1.6 & 0.2 & 0.055 & 0.0060 & 0.02 & 0.20 & 0.02 & 0.15 & 0.0032 &
1.88\tabularnewline
0.05 & 1.6 & 0.2 & 0.055 & 0.0087 & 0.02 & 0.20 & 0.02 & 0.15 & 0.0027 &
3.22\tabularnewline
0.05 & 1.6 & 0.2 & 0.055 & 0.0083 & 0.02 & 0.20 & 0.02 & 0.15 & 0.0017 &
4.88\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table 2} The abbreviated chemical composition of the steels
(wt\%).\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
& \textbf{C} & \textbf{Mn} & \textbf{Si} & \textbf{Nb} & \textbf{Ti} &
\textbf{Mo} & \textbf{N}\tabularnewline
\midrule
\endhead
\textbf{Low-Ti} & 0.045 & 1.6 & 0.22 & 0.052 & 0.0060 & 0.15 &
0.0032\tabularnewline
\textbf{High-Ti} & 0.045 & 1.6 & 0.22 & 0.052 & 0.0087 & 0.15 &
0.0017\tabularnewline
\bottomrule
\end{longtable}

The Fe-C phase diagram for the Low-Ti steel up to 0.1 wt\% C
(\textbf{Figure 1} ) indicates that TiN precipitates form in the
austenite matrix of Low-Ti steel at approximately 1650 K regardless of C
concentration, while the Ti carbonitride precipitates form at much lower
temperatures. The precipitation temperature of Ti carbonitrides has been
increased significantly to \textasciitilde{} 1330K with carbon
concentration up to 0.02 wt\% and then gradually increased to
\textasciitilde{}1440K with further addition of carbon up to 0.1 wt\%.
It indicates that the variation in carbon concentration around 0.05-0.2
(typical carbon concentration of linepipe steels) has only a slight
affect on the precipitation of nitrides in these steels.

The precipitation of TiN precipitates occurs at much higher temperatures
than carbonitrides, where the alloying elements of Ti and N diffuse much
faster than at lower temperatures {[}12{]}. It suggests that TiN
precipitates form larger size precipitates than carbonitrides in the
matrix.

3.1. The effect of titanium and nitrogen on the precipitates of low
carbon, low alloyed steels

\textbf{Figure 2} shows the phase diagram of Low-Ti steel as a function
of N concentration. Although the Ti concentration remains constant over
the whole composition range, the precipitation temperate of titanium
nitrides has been increased substantially with up to 0.001 wt\% N, and
gradually increased with further N addition. The precipitation
temperature of Ti carbonitrides shows only a slight deviation with the
variation in N concentration, indicating that carbonitride precipitates
mainly contain carbon.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 1} Fe-C phase diagram for Low-Ti steel showing the
composition dependence of the formation temperature of titanium nitrides
and carbonitrides.

\textbf{Figure 3} compares the formation temperature of TiN precipitates
in Low-Ti (red curves) and High-Ti (blue curves) steels. Higher Ti
concentration increases the temperatures of TiN precipitation. It should
be noted that the Ti/N ratio varies considerably with the Ti content. At
a given Ti/N ratio, Ti nitrides precipitate in the High-Ti steels at
temperatures approximately 70K higher than the Low-Ti steels.
Considering the effect of diffusion on the precipitates size, TiN
precipitates which form at lower temperatures are anticipated to be
smaller in size. A study {[}19{]} on the effect of weld thermal cycles
on precipitates of Ti-Vanadium (V)-microalloyed steels with Ti/N ratios
of 0.8-1.8, indicated that only TiN precipitates survived high energy
weld cycles and determined the Austenite grain size. Smaller, coherent
precipitates with a higher concentration of V than Ti formed in these
steels, but dissolved during the welding process. This is in agreement
with the findings of this study, confirming that carbonitride
precipitates which form at lower temperatures, are smaller in size and
mostly dissolve during heat treatments, although the time is as limited
similar to welding methods.

The optimum Ti/N ratio is found to be around stoichiometry {[}1, 9, 11,
20{]}; however, high contents of Ti and N promote the formation of
coarse TiN precipitates which deteriorate toughness {[}1, 20{]}.
Therefore, a reduced content of these elements is suggested while
simultaneously retaining a minimum Ti/N ratio to improve the mechanical
properties of these steels {[}1{]}. The presence of V, Nb, Molybdenum
(Mo), Zirconium (Zr) and Boron (B) nitride forming elements in the
microalloyed steels might vary the optimum Ti/N ratio due to interaction
of N with a combination of these elements {[}20{]}. Therefore,
calculated Fe-N phase diagrams for these alloys provide advice on the
optimum Ti/N ratio.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 2} Nitrogen dependence transformation temperature of
Low-Ti steel\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 3} Comparison of formation temperatures of precipitates
in a Low-Ti (red curves) and a High-Ti (blue curves) steel as a function
of nitrogen concentration, demonstrating that titanium nitride
precipitates form at higher temperatures and lower nitrogen content in a
High-Ti steel.

\textbf{3.2. The effect of niobium on the precipitates of low carbon,
low alloyed steels}

Table 3 presents chemical compositions of a few Nb-microalloyed linepipe
steels fabricated in industries, adopted to study the effect of Nb
concentration on the formation temperature and composition of nitride
and carbonitride precipitates. There is a substantial deviation in the
Manganese (Mn), Silicon (Si), Mo and V content of the selected steels.
In order to fully realise the effect of these variations on the nature
of precipitates, the phase diagrams of an abbreviated composition of a
steel named High-Nb in Table 4, was calculated by varying Mn, Si and Mo
composition within the range shown in the second row of Table 4. The
steel with chemical composition of ''High-Nb'' in table 3 represents
steels 2, 3 and 4 in table 3 and ''Low-Nb'' is representative of steel 1
in Table 3.

Interestingly, the deviations of Mn, Si and Mo contents within the
designated composition range in Table 4, show no variation in the
formation temperatures of carbides and nitrides (\textbf{Figure 4a} ).
The precipitation temperatures of nitrides and carbonitrides as a
function of Mo content for the High-Nb steel are shown in \textbf{Figure
4(a)} , as a model of obtained phase diagrams for these elements. It is
worth noting that nitride and carbonitride precipitates contain both
elements of Ti and Nb in these steels, specified by the calculated phase
diagrams. Experimental microscopy analysis of rolled Ti-Nb microalloyed
steels with similar compositions of the current study, presented
ferrite-bainite microstructure which included precipitates of (Ti, Nb)N,
120-400nm in size, spherical or plate-like precipitates of (Nb, Ti)C
within 10-120nm and needle-like 3-5nm (Nb, Ti)C precipitates {[}21{]}.
Carbide precipitates are believed to be carbonitrides because the method
was unable to analyse C and N in small precipitates {[}20{]}. Nitride
precipitates which form at higher temperatures form larger precipitates
than carbonitride precipitates.

\textbf{Figure 4(b)} shows that although the variation of V content has
no effect on the formation temperature of nitride precipitates, it
introduces vanadium carbonitride precipitates to the alloy at
temperatures below \textasciitilde{}1050K. It indicates that the
composition, size and distribution of the carbonitride precipitates in
steel 2 and 3 (table 3), is expected to be distinguishable from the rest
of the alloys in table 3. The solubility limit of V in austenite is much
greater than Ti and Nb in microalloyed steels, and therefore remains in
solution during heat-treated processing in the austenite range to a much
greater extent. V carbonitride precipitates have a thin, fibrous
morphology and contribute to strengthening {[}20{]}.

\textbf{Table 3} Example Chemical composition of Nb -contained line
pipesteels (wt\%).\selectlanguage{english}
\begin{longtable}[]{@{}lllllllll@{}}
\toprule
\textbf{Steel} & \textbf{C} & \textbf{Mn} & \textbf{Si} & \textbf{Nb} &
\textbf{Ti} & \textbf{V} & \textbf{Mo} & \textbf{N}\tabularnewline
\midrule
\endhead
\textbf{High Nb} & 0.05 & 1.5 & 0.16 & 0.11 & 0.012 & 0.003 & 0.002 &
0.0048\tabularnewline
\textbf{Composition Ranges} & & 1.2-1.6 & 0.16-0.3 & & & 0-0.03 & 0-0.03
&\tabularnewline
\textbf{Low Nb} & 0.14 & 1.2 & 0.27 & 0.001 & 0.014 & 0.006 & - &
0.0021\tabularnewline
\bottomrule
\end{longtable}

\textbf{Table 4} The chemical composition of Nb -contained steels in
mass fraction. The second row shows the composition range of elements
selected to assess the variations.

\textbf{Figure 5} shows the phase diagram of the High-Nb steel as a
function of carbon concentration, demonstrating that titanium nitride
precipitates in steels with carbon concentrations \textless{}0.06 wt\%
form at temperatures \textasciitilde{}50K higher than the steels with
higher carbon concentrations. Whereas niobium carbonitride precipitates,
form at much lower temperatures in low carbon steels. It suggests that
titanium nitride precipitates in low carbon steels are expected to be
larger in size than in alloys with higher concentrations of carbon
(above 0.06 wt\%). Steel 4 in Table 3 contains 0.05 wt\% C and therefore
it is anticipated to include larger TiN precipitates than the other
alloys. The formation temperature of Nb carbonitrides is increased
considerably to \textasciitilde{}1400K by carbon concentration up to
0.02 wt\% and then gradually increased to \textasciitilde{}1520K at 0.1
wt\% carbon. The niobium carbonitride precipitates contain higher ratios
of carbon to nitrogen and also contain a small amount of titanium. The
elemental fractions change by temperature and carbon concentration.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 4} (a) Molybdenum; (b) Vanadium dependence formation
temperatures of carbide and carbonitride precipitates, calculated for
High-Nb steel (table 4).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure 5} Fe-C phase diagram for High-Nb steel, showing the
formation temperature of titanium nitrides and carbonitrides varies
significantly by carbon concentration.

The variation of niobium and nitrogen concentrations on the
transformation and precipitation temperatures, have been studied for the
High-Nb steel. The niobium concentration up to 0.12 wt\% has no
influence on the precipitation temperature of nitrides in this steel
(\textbf{Figure 6(a)} ), whereas the formation temperature of niobium
carbides is increased considerably up to 0.03 wt\% Nb. Further addition
of niobium gradually increases the precipitation temperature of
carbides. The calculated chemical composition of precipitates at 1400K
and 1300K are compared for steels with niobium concentrations of 0.03
and 0.11 wt\% respectively in table 5. The nitrides contain mostly
titanium, and niobium forms carbides.

A study of non-isothermal austenite grain growth, in the Heat Affected
Zone (HAZ) of an X80 microalloyed steel, has shown that the increasing
peak temperature from 1223 K to 1523 K results in a significant growth
of the grain size {[}8{]}. The chemical composition of the studied alloy
is similar to the High-Nb steel of the current report with 0.034 wt\%
Nb. The calculated phase diagram in \textbf{Figure 6(a)} indicates that
the niobium carbonitride precipitates are only partially dissolved at a
peak temperature of 1223K, and therefore remaining precipitates suppress
the austenite grain growth via pinning of the grain boundaries. Whereas
austenite grains can grow rapidly at a peak temperature of 1523K, where
niobium carbonitrides are dissolved completely in the matrix. Titanium
nitride precipitates are formed at temperatures above 1700K, and remain
undissolved in the austenite matrix at both selected peak temperatures.
The nitride precipitates which form at high temperatures are generally
large in size and therefore, are not expected to contribute considerably
in suppressing the austenite grain coarsening {[}5, 6{]}.

\textbf{Table 5} The chemical composition of precipitates in the High-Nb
steel for nominated niobium concentrations.\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
\textbf{Temperature} & \textbf{Nb concentration in the steel (wt\%)} &
\textbf{Precipitates concentration (at\%)} & \textbf{Precipitates
concentration (at\%)} & \textbf{Precipitates concentration (at\%)} &
\textbf{Precipitates concentration (at\%)} & \textbf{Precipitates
concentration (at\%)}\tabularnewline
\midrule
\endhead
& & \textbf{Ti} & \textbf{Nb} & \textbf{N} & \textbf{C} & \textbf{Other
elements}\tabularnewline
1400 & 0.03 & 48 & 2 & 47 & - & 3\tabularnewline
1300 & 0.11 & 48 & - & 45 & 5 & 2\tabularnewline
& & - & 47 & 7 & 42 & 4\tabularnewline
\bottomrule
\end{longtable}

The formation temperatures of precipitates in the Low-Ti (red curves)
steel are compared with the High-Nb steel (purple curves)
in\textbf{Figure 6(b)} as a function of nitrogen
concentration.\textbf{Figure 3} showed that increasing the concentration
of titanium increases only the precipitation temperature of titanium
nitrides, however, \textbf{Figure 6(b)} shows that the addition of
niobium has significantly increased the precipitation temperatures of
both nitrides and carbides. This will result in more thermally stable
precipitates, which contribute to suppressing austenite grain growth
during heat treatments. It can explain the fine grain size distribution
in niobium-alloyed steels {[}7, 8{]}.

\textbf{Figure 6(b)} also demonstrates that the precipitation
temperature of niobium carbides is almost independent of the nitrogen
concentration and the precipitation temperatures are increased with the
niobium concentration (\textbf{Figure 6(a)} ). Therefore, it is expected
that the niobium carbide precipitates which form in steel 2 (Table 3)
appear larger in size than the precipitates in steel 4, originated from
precipitation at higher temperatures.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Figure 6} (a) The phase diagram of the High-Nb steel as a
function of niobium concentration; (b) Overlapped phase diagram of the
Low-Ti (red curves) and High-Nb (purple curves) steels

The phase diagrams were calculated for the Low-Nb steel (\textbf{Table
4} ) separately due to a much higher concentration of carbon in steel 1
than the rest of the alloys in Table 3. The transformation and
precipitation temperature of the Low-Nb steel as a function of carbon
and niobium concentration are shown in \textbf{Figure 7} . Similar to
the High-Nb steel, the precipitation temperature of niobium carbides is
increased with niobium concentration.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}

\textbf{Figure 7} The phase diagram of the Low-Nb steel as a function of
(a) carbon and (b) nitrogen concentration

The Fe-C phase diagrams of the low-Nb steel (pink) and High-Nb steel
(blue) are compared in \textbf{Figure 8} . The concentration of nitrogen
(0.0021 wt\%) and niobium (0.001 wt\%) in the Low-Nb steel, is lower
than High-Nb steel which contain 0.0048 wt\% and 0.11 wt\% nitrogen and
niobium respectively. This results in precipitation of titanium nitrides
at lower temperatures in the Low-Nb steel compared to the High-Nb steel
with larger nitrogen and niobium concentrations. It suggests that all
precipitates including carbides and nitrides, will dissolve in the
matrix where the Low-Nb steel is heat treated above \textasciitilde{}
1673K. This will cause a rapid austenite grain growth.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

\textbf{Figure 8} Comparison of the Fe-C phase diagram for the Low-Nb
(pink curves) andHigh-Nb (blue curves) steels, showing that low
concentration of nitrogen and niobium in the Low-Nb steel decreases the
precipitation temperatures of titanium nitrides.

\textbf{CONCLUSION}

The ThermoCalc software package, using the thermodynamic databases of
TCFE7 was employed to study the precipitation of nitrides and
carbonitrides in common microalloyed line pipe steels.

- Higher concentration of titanium increases the precipitation
temperatures of titanium nitrides by approximately 70K, suggesting
larger titanium nitrides.

- The niobium concentration up to 0.12 wt\% in a Ti and Nb microalloyed
steel shows no influence on the precipitation temperature of nitrides,
whereas the formation temperature of niobium carbides is increased
substantially up to 0.03 wt\% Nb. The nitride precipitates contain a
high concentration of titanium while carbides mostly formed of niobium.

- Titanium concentration increases the precipitation temperatures of
titanium nitrides in a Ti and Nb microalloyed steel. However, the
precipitation temperatures of both nitrides and carbides increase
significantly with Nb concentration.

The results are in good agreement with experimental data in the
literature, highlighting the suitability of primary calculation approach
to the design of new alloys and heat treatment processes of microalloyed
steels.

\textbf{ACKNOWLEDGEMENTS}

The author would like to thank the University of Wollongong and Prof.
Rian Dippenaar for providing access to the ThermoCalc software and the
databases.

\textbf{CONFLICT OF INTEREST}

The author has no conflict of interest relevant to this article.

\textbf{REFERENCES}

1. Shen, Y. and S.S. Hansen, \emph{Effect of the Ti/N ratio on the
hardenability and mechanical properties of a quenched-and-tempered
C-Mn-B steel.} Metallurgical and Materials Transactions A, 28(10): p.
2027-2035, 1997.

2. Mintz, B., \emph{Influence of nitrogen on hot ductility of steels and
its relationship to problem of transverse cracking.} Ironmaking \&
Steelmaking, 27(5): p. 343-347, 2000.

3. Yan, B., et al., \emph{The Effect of Precipitate Evolution on
Austenite Grain Growth in RAFM Steel.} Materials (Basel, Switzerland),
10(9): p. 1017, 2017.

4. Cuddy, L.J. and J.C. Raley, \emph{Austenite grain coarsening in
microalloyed steels.} Metallurgical Transactions A, 14(10): p.
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