Experimental Study of High Temperature Phase Equilibria in the Iron-Rich Part of the Fe-P and Fe-C-P Systems

Abstract

During the solidification of steel, phosphorus strongly segregates in the interdendritic liquid phase. In the continuous casting process, even low levels of P may have a detrimental effect on the final product quality. However, phosphorus is partly added up to 0.10 wt pct to improve the mechanical properties of advanced steel grades nowadays, e.g., High-Strength Interstitial-Free (HSIF). To provide new experimental data for the development of thermodynamic databases and solidification models for P alloyed steel grades, phase equilibria in the Fe-P and Fe-C-P key systems were studied up to 1550 °C using differential scanning calorimetry (DSC) and high temperature laser scanning confocal microscopy (HT-LSCM). Special focus was placed on solid/liquid equilibrium temperatures in the Fe-rich part of the binary Fe-P system between 0.025 and 9 wt pct P. In the ternary system, three isoplethal sections with 0.10 mass pct. P, 0.20 mass pct. C and constant mass percent ratio P/C of 2 were investigated. In the latter section, HT-LSCM observations were linked with DSC signals to optically identify present phase stabilities. Particularly at [pct P] < 1, significant differences between performed measurements and calculated phase equilibrium temperatures using thermodynamic assessments from the literature were identified. In all ternary sections, the experiments indicate less influence of P on the hypo-peritectic range compared to the thermodynamic calculations.

An Assessment of Analytical Liquidus Equations for Fe-C-Si-Mn-Al-P-Alloyed Steels Using DSC/DTA Techniques

“…The typical experimental error in measuring solid-liquid phase transformations with the present setup is suggested to be ± 2°C. [26] As expected, the onset temperature (T S ) is not dependent on the heating rate: T S measured with 10°C/min and the equilibrium data calculated using the Tau-R software are identical. In measuring the peak temperature (T L ), the scanning…”

“…A part of the DSC/DTA dataset has already been published in previous work of the authors [21][22][23][24][25][26] and further liquidus data in the Fe-C-Al and Fe-Si-Mn-Al key-systems are provided in the present study, see Table IV. Additional background results of ternary (Fe-C-Si, Fe-C-Mn, Fe-Si-P, Fe-Al-P), quaternary (Fe-C-Si-Mn, Fe-C-Mn-P), and multicomponent (Fe-C-Si-Mn-Al-P) alloys within the composition range of Table III are considered in the discussion.…”

“…In all DSC experiments, a heating rate (HR) of 10°C/min was applied and samples of 50 mg were used. [26] In the following section an ''onset'' in the DSC signal represents the first deviation from the baseline and corresponds to the solidus temperature T S . A ''peak'' indicates the end of the phase transformation and is defined as T L .…”

“…[29,30] The Tau-R method enables the determination of the equilibrium data from only a single DSC experiment. At this point, the authors refer to their previous work [26] regarding more detailed information on the Tau-R method and its successful application to characterize melting equilibria in steel using DSC.…”

“…Note, that the present work (PW) differs from Miettinen and Howe's formula [14] only in the modified P coefficient of ferrite. T L in references [21,25] were measured by DTA (NETZSCH STA409PG Luxx); in references [22][23][24]26] and in the present work a NETZSCH DSC404F1 Pegasus was used. DT…”