On the Capability of Nonmetallic Inclusions to Act as Nuclei for Acicular Ferrite in Different Steel Grades

Abstract

Acicular ferrite nucleates intragranularly on nonmetallic inclusions, forming a microstructure with excellent fracture toughness. The formation of acicular ferrite is strongly affected by the size, content, and composition of nonmetallic inclusions, but also by the composition of the steel matrix. The potential of inclusions in medium carbon HSLA (high-strength low-alloyed) steels has been the main focus in the literature so far. The current study evaluates the acicular ferrite capability of various inclusions types in four different steel grades with carbon contents varying between 0.04 and 0.65 wt pct. The investigated steels are produced by melting experiments on a laboratory scale and subsequent heat treatment in a High-Temperature Laser Scanning Confocal Microscope. Inclusions are exclusively formed by deoxidation and desulfurization reactions. No synthetic particles are added to the melt. The inclusion landscape is analyzed by Scanning Electron Microscopy. Final ductility of the samples is evaluated based on performed tensile tests. Inclusion types in every steel grade are assessed regarding their nucleation potential always considering the interaction with the steel composition, especially focusing on the role of manganese. The effects of (Ti,Al)O x -, MnS-, and MgO-containing inclusions are discussed in detail.

A Unique Bimodal Microstructure Induced by Ultrafine Inclusions to Improve Mechanical Properties of Low‐Carbon Steels Produced by Twin‐Roll Casting

“…The addition of Mn, Si, and Ti was expected to obtain the potential inclusions to act as nuclei for AF. [12] Indeed, these inclusions effectively induced the formation of UFAF in this work (Figure 1c-e). In the current study, an obvious Ti-rich zone was also observed around the inclusion such as a shell together with UFAF.…”

“…In view of the strong deoxidization of Al, in the current study, it was used to form initial dispersed Al 2 O 3 core to induce the heterogeneous nucleation of the other oxides during solidification process (Figure d). The addition of Mn, Si, and Ti was expected to obtain the potential inclusions to act as nuclei for AF . Indeed, these inclusions effectively induced the formation of UFAF in this work (Figure c–e).…”

Effect of Mg on the Evolution of Inclusions and Formation of Acicular Ferrite in La–Ti‐Treated Steels

“…AF usually nucleates on intragranular nonmetallic inclusions. Therefore, the number density, size distribution, and composition of inclusions influence the formation of AF significantly . It is well established that the increment of the dispersed fine particles enhances the formation of AF .…”

“…Therefore, the number density, size distribution, and composition of inclusions influence the formation of AF significantly. [6] It is well established that the increment of the dispersed fine particles enhances the formation of AF. [7,8] Nako et al [9] and Oh et al [10] also stated that the AF content was in proportion to the number density of inclusions, whereas the types of inclusions are the dominant factors for the nucleation of AF and the composition of inclusions affect the AF formation mainly by the mechanisms, [8] such as 1) the lattice mismatch between the inclusions and matrix metal and [9,11,12] 2) Mn-depleted zone (MDZ) near the inclusions.…”

Cross-Scale Modeling of MnS Precipitation for Steel Solidification

“…The lower modulus and hardness of MnS inclusions compared to those of a matrix give rise to a drop in the fatigue strength of steel [4], while the machinability is amended simultaneously [5]. MnS can also refine the matrix grains by suppressing austenite growth with the pinning effect of MnS-rich precipitates [6] and facilitate acicular ferrite nucleation in the coarse-grained zone so as to promote fractions of the high-toughness phase [7,8]. Many works have been conducted from a variety of perspectives, such as thermodynamics, kinetics, solute segregation and nucleation, in order to determine MnS precipitation to better control its impact on steel performance.…”