Precipitation of titanium nitride in low‐alloyed steel during solidification

Abstract

The microsegregation of titanium and manganese and the precipitation of titanium nitride during dendritic solidification were investigated on an Fe‐0.051%Ti‐0.77%Mn alloy. Carbon and nitrogen were introduced into the liquid sample via the gas phase. The material solidifies as delta ferrite at a carbon mass content of about 0.08% but predominantly as austenite at ≥ 0.3%. At ferritic solidification, an about 3fold enrichment of the titanium in the residual melt between secondary dendrite arms is attained by segregration, whereas in the corners between some dendrites or grains in maximum a 10fold enrichment was detected. TiN particles of 0.5 to 1.0 μm size are formed only in the regions of maximum segregation where the supersaturation is higher than 15fold. An equal supersaturation has been calculated for homogeneous nucleation in the melt. The number of particles is moderate at 0.016% nitrogen but low at 0.008%. At austenitic solidification, the titanium content in the residual melt increases to more than the 30fold of the starting value. Particles formed in the melt grow to diameters in the 2 μm range.

Prediction of Inhomogeneous Distribution of Microalloy Precipitates in Continuous-Cast High-Strength, Low-Alloy Steel Slab

“…The partitioning of microalloying elements (Nb, Ti, and V) predicted from the Clyne and Kurz model [23] at SS and MT locations ( Figure 10) show that microsegregation becomes severe with an increase in depth below the SS. Following the previous studies, [21,32] the composition in solid corresponding to solid fraction, f s~0 .05, and the composition in the liquid corresponding to f s~0 .95, are assumed to be the compositions at the middle of solute-depleted (dendrite center) regions and solute-rich (interdendritic) regions, respectively. The concentration of alloying elements predicted from the Clyne and Kurz model [23] at interdendritic and dendrite center regions at the QT location in the slabs is listed in Table V. The difference between predicted and measured concentrations (Tables III and V) can be caused by the fact that the Clyne and Kurz model [23] predicts the solute partitioning during solidification without considering the homogenization taking place during the subsequent cooling of the slabs from solidus temperature to ambient temperature.…”

“…Previous studies [32,38,39] discussed in detail the modification of Eq. [9] for predicting the TiN precipitation in liquid steel.…”

“…Interfacial energy r~0.8 J/m 2 can be used for TiN precipitation in the liquid. [32,39,40] From the previous equations, it is evident that a different level of supersaturation-resulting from microsegregation-in different regions of solidifying and solidified steel can result in different chemical driving forces (DG v ) for precipitation between those regions. In the interdendritic region, higher DG v will increase the nucleation rate I and reduce the critical nucleus size (r*).…”

Effects of Titanium Addition on Precipitate and Microstructural Control in C-Mn Microalloyed Steels.

“…The above compositional variation predicted by the thermodynamic analysis is a consequence of the continuous mutual solubility of the carbides and nitrides formed by Ti, Nb and V and of the decreasing solubility of the different carbides and nitrides with temperature in the giron. [1][2][3][4][5] …”

“…A proper design of the carbonitride precipitation sequence through control of composition, solidification and cooling rate is therefore of great importance and it was the subject of a large number of research works in the last decades. [1][2][3][4][5][6][7] During recent years, particular attention was dedicated to optimisation of solidification conditions and avoidance of transversal and edge cracking, especially during continuous casting. From hot tensile testing and from industrial practice on microalloyed steel, it was established that a loss of ductility is often encountered at temperatures approximately ranging from 750 to 950°C.…”