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)Ox-, MnS-, and MgO-containing inclusions are discussed in detail.

By increasing the amount of acicular ferrite (AF) in the microstructure, steel toughness can be improved significantly. The steel composition, cooling rate, non-metallic inclusions and austenite grain size have a strong influence on the formation of AF. The present paper describes and compares two approaches to study AF formation in a titanium-deoxidised high-strength low-alloyed steel and its influencing factors on laboratory scale: route A simulates the formation of AF after heat treatment; route B simulates the formation directly after solidification of the melt. The formation of AF is essentially influenced by the former processing, which also changes the optimum cooling parameters substantially. (Ti,Mn)xOy and (Ti,Al,Mn)xOySz are the predominant active inclusion types in the investigated steel.

Die Azikularferritbildung ist durch die Stahlzusammensetzung, Kühlrate, Austenitkorngröße und Einschlusslandschaft bestimmt, jedoch ist deren exakte Wirkung noch nicht restlos geklärt. Im Rahmen des FWF-Projekts TRP 266-N19 wurde eine systematische Methodik zur Untersuchung dieser Einflussfaktoren sowie deren Wechselwirkungen entwickelt. Die vorliegende Arbeit demonstriert die Anwendung dieser Methodik beispielhaft für die Untersuchung des Einflusses von Kühlrate und Bor auf die Azikularferritbildung in einem niederkohligen HSLA-Stahl.

A thermodynamic model coupling microsegregation and inclusion formation using one ChemSage data file is proposed. The thermodynamic equilibrium is calculated using ChemApp to determine the liquidus temperature, solute partition coefficients at the solidification interface and inclusion formation in the residual liquid. During the calculations, solute enrichment is predicted using Ohnaka's model. The logarithm of the coupling microsegregation and inclusion formation is tested through an overall mass balance. With the proposed model, inclusion formation is predicted for the case of medium carbon steels alloyed with titanium and aluminum. The predicted types and compositions of inclusions agree well with the experimental results. Based on the predictions and measurements, the inclusion behavior during solidification is discussed.

Since the 1970s comprehensive effort was spent on the explanation of the acicular ferrite formation. Acicular ferrite (AF) nucleates intragranularly on non-metallic inclusions. The needle or lenticular shaped plates radiate in various directions and create a chaotic, interlocking microstructure. The growth of acicular ferrite grains is diffusionless, but excess carbon is rejected to the remaining austenite shortly after transition. The carbon enriched austenite transforms during the ongoing cooling process to perlite, bainite or martensite, or remains as retained austenite in the final structure. The created multiphase microstructure provides excellent mechanical properties, most notably toughness, so that acicular ferrite steels are of increasing interest for steel producers. In literature four main nucleation mechanisms are described: destruction of the crystal structure, creation of dislocation arrays, reduction of lattice mismatch and chemical changes in the local matrix. Literature suggests that a combination of at least two effects is responsible for the nucleation of acicular ferrite, but the exact impact of the mechanisms is not completely understood yet.

Acicular ferrite is a microstructure nucleating intergranularly on non-metallic inclusions and forming an arrangement of fine, interlocking grains. This structure is known to improve steel properties, especially steel toughness, essentially. The formation of acicular ferrite is mainly affected by steel composition, cooling rate, inclusion landscape and austenite grain size. In recent decades, extensive research has been conducted to investigate these factors. The present paper provides an overview of the impact of published results and the state of knowledge regarding acicular ferrite formation. Special attention is paid to the effect of carbon, manganese and titanium addition to steel, as well as the optimum size, number and composition of non-metallic inclusions. In addition, the reactions during the nucleation and growth of acicular ferrite needles are briefly addressed. Further, characteristics of acicular ferrite and bainite are summarized, which should help to distinguish these similar structures.

By using a Laser Scanning Confocal Microscope combined with a High Temperature Furnace (HT-LSCM) for the in situ investigation of acicular ferrite (AF) formation in HSLA steels, new information about the mechanism of formation of this high toughness phase can be gained. Due to the utilization of an inert furnace atmosphere, the ability to accurately adjustment the austenitizing temperature and the well-controllable cooling conditions, the interactions between steel composition, austenite grain size, cooling rate and the fraction of AF formed have been analyzed in detail. The present work focuses on necessary adaptions and appropriate settings of the HT-LSCM for the investigation of the formation of AF. An intensive study of setting parameters for the visualization of the AF formation is done. Special attention is paid to the techniques by which austenite grain size is determined and the effect of austenite grain size on the fraction of AF formed. In addition, the complexity of studying manganese alloyed steels in the HT-LSCM has been elucidated.

Titanium-alloyed ferritic chromium steels are a competitive option to classical austenitic stainless steels owing to their similar corrosion resistance. The addition of titanium significantly influences their final steel cleanliness. The present contribution focuses on the detailed metallographic characterization of titanium nitrides, titanium carbides and titanium carbonitrides with regard to their size, morphology and composition. The methods used are manual and automated Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy as well as optical microscopy. Additional thermodynamic calculations are performed to explain the precipitation procedure of the analyzed titanium nitrides.The analyses showed that homogeneous nucleation is decisive at an early process stage after the addition of titanium. Heterogeneous nucleation gets crucial with ongoing process time and essentially influences the final inclusion size of titanium nitrides. A detailed investigation of the nuclei for heterogeneous nucleation with automated Scanning Electron Microscopy proved to be difficult due to their small size. Manual Scanning Electron Microscopy and optical microscopy have to be applied. Furthermore, it was found that during solidification an additional layer around an existing titanium nitride can be formed which changes the final inclusion morphology significantly. These layers are also characterized in detail.Based on these different inclusion morphologies, in combination with thermodynamic results, tendencies regarding the formation and modification time of titanium containing inclusions in ferritic chromium steels are derived. (C) 2014 Elsevier Inc. All rights reserved.

Specific types of non-metallic inclusions are known to act as heterogeneous nuclei for the formation of acicular ferrite, which provides excellent toughness. By increasing the amount of acicular ferrite in the microstructure, the properties of HSLA steels can be optimized significantly.Although the formation of acicular ferrite caused by heat treatments (thermomechanical treatments or welding) is quite well described in literature, there is less information to find about the formation of acicular ferrite immediately out of the liquid melt. Within the present study experiments on laboratory scale are carried out simulating the influence of cooling conditions and Ti-content on size, chemical composition and morphology of non-metallic inclusions and consequently on the amount of acicular ferrite. All experiments were carried out with a dipping test simulator enabling very well controllable cooling conditions. Optical microscopy in combination with special etching methods as well as SEM/EDS-analysis was used for microstructure and inclusion characterization.