The effects of non-metallic inclusions in nucleating acicular ferrite in steels during cooling from a weld or cooling from an austenitic temperature are reviewed. The influence of the acicular ferrite (AF) structure on mechanical properties of steels such as strength and toughness is briefly mentioned. The different factors affecting the formation of acicular ferrite, such as the soluble content of alloying elements in steel, cooling rate from austenitizing temperature, austenite grain size and inclusion characteristics in steel, are discussed. The mechanisms of acicular ferrite formation on non-metallic inclusions, such as reduction of interfacial energy, mismatch strain between the inclusion and ferrite/austenite, thermal strains at the inclusions and changes in matrix composition near the inclusions are also discussed. Finally, the effects of inclusion characteristics, such as size, number and composition are described and their effectiveness in nucleating acicular ferrite is discussed.

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On the Role of Non-metallic Inclusions in the Nucleation of Acicular Ferrite in Steels

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.

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Acicular Ferrite Formation and Its Influencing Factors - A Review

Controlled rolling followed by two-stage continuous cooling was carried out in-house to study the microstructure and mechanical properties of a low carbon microalloyed steel plate of medium thickness gauge. The objective of the study was to develop a process of obtaining excellent mechanical properties (strength, toughness, and ductility) in a microalloyed steel. The process is industrially viable because it does not require high degree of cooling and large reduction during thermo-mechanical processing. The study demonstrates that it is possible to obtain yield strength, tensile strength, elongation, and percentage reduction in area 585 MPa, 680 MPa, 29.5%, and 55%, respectively, with total deformation reduction of 50% and small first stage cooling rate. The total impact energy at −20 °C was 140 J. The microstructure consists of polygonal ferrite and acicular ferrite. The strain-induced 20–30 nm precipitates act as nucleation sites for acicular ferrite. The acicular ferrite contributes to high strength and good toughness, while polygonal ferrite provides excellent elongation.

Structure–mechanical property relationship in low carbon microalloyed steel plate processed using controlled rolling and two-stage continuous cooling

Samples of medium carbon steel were examined after heating between 900℃ - 980℃ and soaked for 45 minutes in a muffle furnace before quenching in palm oil and water separately. The mechanical behavior of the samples was investigated using universal tensile testing machine for tensile test and Vickers pyramid method for hardness testing. The microstructure of the quenched samples was studied using optical microscope. The tensile strength and hardness values of the quenched samples were relatively higher than those of the ascast samples, suggesting improved mechanical properties. However, samples quenched in palm oil displayed better properties compared with that of water-quenched samples. This behavior was traced to the fact that the carbon particles in palm oil quenched samples were more uniform and evenly distributed, indicating the formation of more pearlite structure, than those quenched in water and the as-received samples.

/pdf/evaluation-of-mechanical-properties-of-medium-carbon-steel-49zqdftsdj.pdf

Evaluation of Mechanical Properties of Medium Carbon Steel Quenched in Water and Oil

Investigation on the behaviour of medium carbon and vanadium microalloyed steels by hot forging test

http://digital.csic.es/bitstream/10261/79425/4/7_SM_capdevila.pdf

C Garcı́a-de Andrés

Papers

Acicular ferrite formation in a medium carbon steel with a two stage continuous cooling