The Mechanism of Intragranular Acicular Ferrite Nucleation Induced by Mg-Al-O Inclusions

Abstract

The features of inclusion and microstructure for carbon structural steel containing Mg-Al-O inclusions were studied through the scanning electron microscope (SEM) and Energy Dispersive Spectrometer (EDS). It can be seen that, in Mg-Al-O inclusions, the elements of Mn, Si, and S coexist, and their central mole ratio of Mg/Al varies in a wide range. This value for most inclusions is larger than 0.5, which suggests the formation of solid solution between MgAl2O4and MgO. After etching, the typical microstructure of intragranular acicular ferrites is observed, which is due to the nucleation effect induced by Mg-Al-O inclusions. From the SEM-EDS mapping images, it is found that the element of sulfur accumulates on the periphery of nucleation inclusion. Moreover, line EDS analysis hints that Mn-depletion zone (MDZ) exists in steel matrix, which is adjacent to the complex inclusion. Combined with the theoretical analysis, this phenomenon can be explained by the absorption of Mn due to the magnesium vacancy in MgAl2O4, and this MDZ promotes the nucleation of intragranular acicular ferrite. Through statistical analysis of SEM images for microstructure, the probabilistic nature of inducing nucleation effect is revealed. These results may be helpful to clarify the nature of oxide metallurgy.

Effect of Adding Cerium on Microstructure and Morphology of Ce-Based Inclusions Formed in Low-Carbon Steel

“…Having cerium sulfide has been effective for the formation of AF. Our results are in agreement with other researchers’ 1 2 3 4 5 6 7 8 9 10 11 12 . Figure 4f is related to sample No.…”

“…Many researchers have investigated on improving the behavior of non-metallic inclusions during solidification of steel. It was found that depending on the chemical composition and their size, they can have different impact on properties of steel, starting from strongly negative to almost conditioning obtaining desired mechanical properties 1 2 3 4 .…”

The Relationship between MnS Precipitation and Induced Nucleation Effect of Mg-Bearing Inclusion

“…For example, about the origin of MDZ around Tibearing particles, there are still two viewpoints: (1) the MDZ is produced by MnS precipitation on the Ti 2 O 3 inclusion during cooling process [10]; (2) Ti 2 O 3 can absorb Mn atoms from the steel matrix [11]. Our previous studies also indicate that MnS precipitates on Mg-bearing inclusion [7]. Thus, what is the relationship between MnS precipitation and IAFs nucleation?…”

“…In 2015, our group has proposed Mn-depletion zone (MDZ) theory to explain the mechanism of IAF nucleation induced by Mg-Al-O inclusions [7]. This is based on the following experimental and theoretical evidences.…”

“…The same tool has been utilized by Wang Xinhua et al [8] and Song bo et al [9] to qualitatively prove the existence of MDZ in Rare earth-bearing inclusion and Ti-Mg inclusion. On theoretical side, the vacancy property and crystal structure of MgAl 2 O 4 are discussed to prove that it's potential to accommodate Mn ions [7].…”