Effect of Cooling Rate on AlN Precipitation in FeCrAl Stainless Steel During Solidification

metals Article Effect of Cooling Rate on AlN Precipitation in FeCrAl Stainless Steel During Solidification Zhenqiang Deng 1,2 , Yang He 1 , Jianhua Liu 1, *, Baijun Yan 3 , Yindong Yang 2 and Alexander McLean 2, * 1 2 3 * Institute of Engineering Technology, University of Science and Technology Beijing, Beijing 100083, China; (Z.D.); (Y.H.) Department of Materials Science and Engineering, University of Toronto, Toronto, ON M5S 3E4, Canada; Department of Physical Chemistry of Metallurgy, University of Science and Technology Beijing, Beijing 100083, China; Correspondence: (J.L.); (A.M.); Tel.: +86-010-62332958-6421 (J.L.); +1-416-978-1291 (A.M.) Received: 12 September 2019; Accepted: 8 October 2019; Published: 11 October 2019



 Abstract: The effect of cooling rate on the evolution of AlN inclusions precipitated during solidification in FeCrAl stainless steel was investigated using an experimental study and thermodynamic and kinetic calculations. The number and size of AlN inclusions precipitated under different cooling rates were examined with the feature function of the field-emission scanning electron microscope. A model combining micro-segregation and the diffusion-controlled growth model was set up to determine the mechanism of AlN particle growth. The results showed that AlN precipitates in the mushy zone. The size of AlN particles decreases and the number of AlN particles increases with increasing cooling rate, whereas the volume fraction is essentially unchanged. The AlN particles grow during solidification after the content of solutes in molten steel has exceeded the concentration in equilibrium with AlN. The nitrogen content varies significantly with the cooling rate during solidification. Increasing the cooling rate and reducing the nitrogen content in the molten steel can reduce the AlN particle size in FeCrAl alloys as the growth time decreases. Keywords: FeCrAl stainless steel; AlN; solidification; micro-segregation; diffusion-controlled growth; cooling rate 1. Introduction FeCrAl stainless steel is an ideal material for the production of automotive exhaust gas purifier carriers due to its excellent high-temperature oxidation resistance and low thermal expansion coefficient [1,2]. In order to form a dense aluminum oxide film at high temperatures to obtain good oxidation resistance, the Al content in FeCrAl stainless steel is usually above 3% [3]. The thickness of the steel foil used is only 30–50 µm, and a suitable solidification structure with good plasticity is required in order to ensure reliable yield and surface quality. The composition, size and distribution of inclusions significantly affect the structure and properties of the cast slab [4]. There have been many reports on the characteristics of Al2 O3 and AlN inclusions in regular Al-killed steels [5–8]. In these steels, the dissolved oxygen is very low because of the strong deoxidizing power of Al and thus there is relatively little oxide precipitation during solidification [9]. In the FeCrAl stainless steel, AlN inclusions are the dominant precipitates [10]. These inclusions cannot play an effective role in the heterogeneous nucleation of ferrite and can also cause embrittlement and induce cracking phenomena such as rock candy fracture during continuous casting and hot rolling [11–13]. Some authors [14,15] Metals 2019, 9, 1091; doi:10.3390/met9101091 www.mdpi.com/journal/metals Metals 2019, 9, 1091 2 of 13 report that the AlN precipitates cause low hot ductility by inhibiting dynamic recrystallization and by grain boundary pinning, which is considered to facilitate void formation. Thus, it is important to control the precipitation of AlN inclusions in FeCrAl stainless steel. Most of the inclusions formed during the deoxidation of steel are removed in the process of argon stirring in the ladle and the remainder should be removed in the tundish. Ideally, there should be almost no inclusions in the steel during the casting process [16]. However, in the mold, inclusions can form in the molten steel for two reasons: the equilibrium solubility product decreases with the decrease in temperature, and the solutes are redistributed at the solidification front. During the solidification of steel, micro-segregation results in an enrichment of solutes in the liquid phase, which can lead to the formation and growth of the inclusions [17]. Han et al. [18] found that during solidification, AlN precipitated after the solid fraction reached 0.85 in FeCrAl stainless steel which contained 0.02–0.06 weight percent Ti. To gain a deeper understanding of the formation and growth of inclusions in steel, thermodynamics and kinetics have been widely applied for the evaluation and control of inclusion formation during casting. There are three mechanisms contributing to the growth of inclusions: diffusion-controlled growth, collisions and coarsening [19–21]. Many experimental and mathematical studies [22–28] have been carried out on micro-segregation and inclusion precipitation during solidification. However, in most models, the micro-segregation of solute elements and the precipitation of inclusions were calculated separately, which tends to overestimate the degree of micro-segregation and the amount of inclusions. Thus, coupled micro-segregation and thermodynamic models are advantageous for the precise evaluation of inclusion formation during solidification. The cooling rate is one of the critical parameters in the solidification process which has an important influence on the formation of crystal grains, the growth of crystals, and the size and distribution of precipitates. Controlling the cooling rate to obtain optimal microstructure and precipitates is of great significance with respect to the optimization of the controlled rolling process and subsequent cooling of the steel sheet. In the present work, the cooling rate was varied during unidirectional solidification to determine the effect on the precipitation of AlN in FeCrAl stainless steel. The morphology, number density and size distribution of AlN particles were quantitatively analyzed in samples solidified under different cooling rates. The precipitation of AlN was also predicted using a micro-segregation model combined with a diffusion-controlled growth model. Based on relevant solidification parameters, the enrichment of solutes in molten steel and the size of the AlN particles formed during solidification have been calculated. The results predicted from the model have been compared with observations of inclusions in the FeCrAl alloy samples. 2. Materials and Methods An FeCrAl alloy ingot was produced in a 25 kg vacuum induction furnace with commercial purity materials. The chemical composition of the alloy is given in Table 1. Table 1. Chemical composition of the FeCrAl alloy. Chemical Component C Si Mn Cr P S N O Al Fe + Other wt.% 0.022 0.13 0.20 19.90 0.0058 0.0045 0.0088 0.0020 4.07 balance Rod sample (...truncated)