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Page 2 of 25 Park et al. J Mater Inf 2023;3:5 https://dx.doi.org/10.20517/jmi.2022.37

differential scanning calorimetry, electromagnetic levitation, and contact angle measurement. The present
thermodynamic model has higher accuracy in predicting the solubility of Sn in the body-centered cubic (bcc),
compared to previous assessments. This is achieved by re-evaluating the Gibbs energies of the FeSn and FeSn 2
compounds and the peritectic reaction related to Fe Sn . Also, the inconsistencies related to the miscibility gap
5
3
around X = 0.31-0.81 were resolved. The database developed in the present study can contribute to the
Sn
development of a large CALPHAD database containing tramp elements.
Keywords: Fe-Sn, thermodynamics, CALPHAD, miscibility gap, contact angle measurement, DSC

INTRODUCTION
Over the decades, there has been an increasing demand for steel due to building, construction, and
transportation development. According to the forecast, worldwide steel consumption will reach 2500
million tons by 2050 . In addition to the rising costs of raw materials required to produce pig iron , the
[1]
[2]
global society is going towards a carbon-neutral state. Consequently, iron and steelmaking industries are
forced to reuse ferrous scrap materials to ensure competitiveness.

A severe issue in melting post-consumer scrap material is the accumulation of tramp elements, e.g., Cu and
Sn, in the liquid steel. These tramp elements are difficult to remove during conventional steelmaking
processes . The main quality issue in high-temperature processes, e.g., continuous casting and hot rolling,
[3-6]
related to Sn and Cu, is forming a liquid Cu-rich phase at the scale-steel interface. The phenomenon can be
briefly summarized as follows: elements like Sn and Cu show less affinity to oxygen than Fe. As Fe gets
selectively oxidized, the relative concentration of Cu and Sn increases, leading to enrichment at the surface
or along the austenite (γ-Fe) grain boundaries. Though the solubility of Cu in austenite is high in the
[7]
temperature range of 1,000-1,300 °C (mole fraction X ~0.07-0.11) , Sn significantly reduces the maximum
Cu
solubility. If the saturation limit is exceeded, a Cu-rich liquid phase will form under oxidizing conditions.
The low-melting liquid may penetrate the steel along the austenite grain boundaries. This phenomenon is
[8]
well-known as liquid metal embrittlement . Further, Sn decreases the liquidus temperature of the Cu-rich
melt and favors its stability down to even lower temperatures.

The chemical interaction between Fe and Sn also plays an important role in conventional Sn production. In
the smelting process, Sn is extracted from SnO (cassiterite). SnO ore is often found with a gangue of Fe O
4
2
2
3
during mining. The reduction of Fe O results in the formation of a Fe-Sn alloy, known as hardhead [9,10] . The
3
4
recirculation of hardhead decreases the furnace capacity and increases the energy consumption in the
[12]
smelting . The need to recover Sn efficiently from secondary resources is now drawing attention .
[11]
Thermodynamic databases have been recently developed by the CALculation of PHase Diagrams
(CALPHAD) method [13,14] for a variety of alloying systems. Within the framework of this approach,
extensive experimental data, either from the literature or as the results of own measurements, are taken into
account to model the stable phases of one, two or three-component systems. The great advantage of self-
consistent CALPHAD databases is their accurate extrapolation to multicomponent alloys and the fast
calculation runtime using available thermochemical software packages [15-17] . Therefore, a reliable description
of the Fe-Sn system can be used in future multicomponent databases to describe the mutual influence of Cu
and Sn on the high-temperature solubility limit in austenite or to predict the thermodynamic behavior of Fe
and Sn in the smelting process.