Search

AISI 416 steel is a free-cutting stainless steel with a banded structure (BS) that is formed by element segregation, and its secondary phase particles significantly affect its microstructure and properties during the hot working process. In this study, we investigated the formation mechanism of the BS and its relationship with secondary phase particles (MnS and M23C6) in 416 steel by carrying out thermodynamic calculations and high-temperature confocal laser scanning microscopy, electron backscatter diffraction, scanning electron microscopy, and electron probe microanalyzer measurements. The BS was identified to be a delta-ferrite (δ-Fe) phase rich in Cr. In addition, when the volume fraction of δ-Fe was the largest during the solidification process (1392 °C, 1665 K) the BS was most likely to precipitate. The length and width of each grain in the BS were approximately 15 to 20 and 5 μm, respectively. The spacing between the BSs was in the range of 30 to 60 μm. The heat preservation treatment (1170 °C, 1443 K) of the steel could barely improve the shape of the BS. The fine M23C6 particles around the BS enhanced the segregation of Cr and worked with the MnS inclusions to pin the surrounding grains in order to prevent their coarsening during the heat preservation process. The metastable banded structure (MBS) formed near the BS transformed into a martensite phase because the concentration of the segregated Cr in the MBS was not sufficient to stabilize its microstructure. The MnS inclusions pinned and refined the grains and showed four types of distributions, namely between two grain boundaries, between polycrystalline boundaries, within the grains, and throughout the grains.

There are many types of non-metallic TiN-based inclusions observed in GCr15 bearing steel, including single-particle TiN, multi-particle polymerized TiN, and complex inclusions like TiN-MnS, TiN-MgO-MgAl2O4 (TiN-MgO-MA), and TiN-MgAl2O4-MnS (TiN-MA-MnS). Thermodynamic calculations suggest that single-particle TiN precipitates dominate the mushy zone of GCr15 bearing steel. Kinetic calculations regarding TiN growth suggest that the final size of the single-particle TiN ranges between 1 and 6 μm in the initial concentration range of [pct Ti] = 0.0060 to 0.0079 and [pct N] = 0.0049 to 0.0070, at 1620 to 1640 K and a local cooling rate of 0.5 to 10 K/s. The multi-particle polymerized TiN are formed by single TiN particles in three stages: single-particle TiN inclusions approach each other drawn by the cavity bridge force (CBF), local active angles consolidate, and neck region sintering occurs. Based on the thermodynamic calculations of TiN, MnS, and MgO precipitation, the formation behaviors of complex inclusions of TiN-MnS, TiN-MgO-MA, and TiN-MA-MnS were investigated.

Nitride and carbide are the second phases which play an important role in the performance of bearing steel, and their precipitation behavior is complicated. In this study, TiN-MCxprecipitations in GCr15 bearing steels were obtained by non-aqueous electrolysis, and their precipitation mechanisms were studied. TiN is the effective heterogeneous nucleation site for Fe7C3 and Fe3C, therefore, MCxcan precipitate on the surface of TiN easily. The chemistry component of MCx consists of M3C and M7C3 (M = Fe, Cr, Mn) and Cr3C2. TiN-MCx with high TiN volume fraction, TiN forms in early stage of solidification, and MCx precipitates on TiN surface after TiN engulfed by the solidification advancing front. TiN-MCx with low TiN volume fraction, TiN and MCx form in late stage of solidification, TiN can not grow sufficiently and is covered by numerous precipitated MCxparticles.

At present, the effective ways to improve the cleanliness of S50C die steel are Ca or Mg-Al treatment processes. In order to explore the effect difference of two kinds of modification process of S50C killed steel, evaluate the industrial application prospect of the two processes, and clarify the modification mechanism. In this paper, the advantages of Mg-Al modification are demonstrated from the aspects of theoretical basis and actual sample modification effect. The thermodynamics and kinetics of inclusion precipitation, composition, morphology, and distribution are analyzed. The results show that: the precipitation temperature of MnS in S50C die steel is 1686 K, the corresponding solid-phase rate is 0.98. In Mg-Al modification, when the Al content is 332 ppm, the Mg content should be controlled below 14.1 ppm. When the Al content is higher than 0.02%, the Ca content should be controlled below 28.7 ppm. Kinetic calculations show that the equilibrium shape size of MnS is in the range of 0.3‑1.4 µm. Both modifications increase the nucleation rate of inclusions and control the shape and size of inclusions by pre-precipitation. Ca treatment is preventing the formation of large inclusions by forming calcium aluminate. Mg can provide more uniform nucleation sites and form smaller inclusions.

It is an important way for TiN inclusions to play the role of oxide metallurgy to induce inclusions or ferrite heterogeneous nucleation, while MnS and Fe precipitate on TiN are two typical growth modes. Understanding the formation process of TiN–MnS/Fe composite interface is extremely important for the control of TiN containing composite inclusions in steel and the effectiveness of catalyst. Based on the thermodynamic characteristics of TiN induced precipitation of MnS and Fe in steel, the bond structure, charge and energy of TiN crystal structure, TiN (111) N terminal surface structure and TiN–MnS/Fe interface structure are calculated by the first principle method. The growth mechanism of MnS/Fe on TiN surface at atomic scale is analyzed. The results show that TiN is a semi-metallic crystal with a band gap of 3.0 eV; Mn and Fe atoms will be adsorbed on the TiN (111) N terminal surface in the form of face-center cubic (FCC) stacking; MnS and Fe will grow on the TiN surface to form an interface system, and the influence distances of TiN template effect are 12.587 A and 10.588 A, respectively. The growth part (MnS and Fe) affected by TiN can be divided into two layers: coherent epitaxial layer (CEL) and transition layer (TL), in which the CEL and TL of TiN–MnS interface are both four layers, while the two and four layers of TiN–Fe interface.

A calculation has been performed to explore the mechanism of aggregation reaction between two small molecular clusters [(Al2O3) n1 and (Al2O3) n2] by the density functional theory method. Five pathways of aggregation reactions between two different (Al2O3)1 clusters isomers were identified. The detailed process of (Al2O3)1 interaction with (Al2O3)2 were also obtained. All the products of the aggregation reactions between two cage structures are cage-dimer structures. We calculated the thermodynamic properties of all the aggregate clusters. The Gibbs free energy changes of aggregation reactions in 0–1000 K were negative and increased with the temperature increase. The energy changes of enthalpy and entropy were also obtained.

In order to control the structure and size distribution of silica inclusions in high purity steel, it is necessary to understand the nucleation mechanism of solid silica in molten steel. In high temperature reactions, crystallization begins with nucleation, which plays a crucial role in determining the structure and size of solid products. Nucleation originates in the formation of product intermediates. The structure and thermodynamic properties of silica clusters as the intermediate of solid silica products during nucleation were calculated by density functional theory. Comparison of thermodynamic properties of silica clusters and silicon deoxidation equilibrium experiment in liquid iron results shows the silica clusters with most of the dissolved silicon and oxygen in equilibrium; the molten iron silicon deoxidation reaction ([Si] [Formula: see text] [O] [Formula: see text] cannot reach thermodynamic equilibrium state, and some deoxidation products could only exist in the form of silica clusters but not the solid silica. Therefore, the nucleation process of solid silica in Fe-O-Si melt can be considered as a two-step process with silica clusters as intermediates. Finally, there are two paths of solid silica inclusion formation: one is that in the molten iron, the dissolved silicon reacts with the dissolved oxygen to form silica clusters, and clusters further nucleate and grow up; the second one is that silica clusters directly crystallized during the cooling process of the melt.