Non-metallic inclusion and intragranular nucleation of ferrite in Ti-killed C–Mn steel

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

Ferrite nucleation potency of non-metallic inclusions in medium carbon steels

The continuous cooling transformation (CCT) behaviors of three low carbon pipeline steels containing the different carbon and alloy additions such as Mn, Nb, V, Ti and/or Mo were investigated in the undeformed and deformed conditions, respectively. The corresponding static (without hot deformation) and dynamic (with hot deformation) CCT diagrams were constructed, which almost involved the formation curves of bainitic ferrite, acicular ferrite, polygonal ferrite, and pearlite. It was found that with the exception of V, the aforementioned alloy additions played a significant role in suppressing the formation of polygonal ferrite and promoting the formation of acicular ferrite. Furthermore, hot deformation could also strongly promote the formation of acicular ferrite, that is, the temperature zone of acicular ferrite transformation was enlarged from 400-600 degreesC in the static CCT diagrams to 450-700 degreesC in the dynamic CCT diagrams. The corresponding cooling rate range of acicular ferrite transformation was significantly increased, and the island constituents in acicular ferrite became finer due to hot deformation. (C) 2003 Elsevier Science B.V. All rights reserved.

Continuous cooling transformation of undeformed and deformed low carbon pipeline steels

Inclusion assisted microstructure control has been a key technology to improve the toughness of C–Mn and low alloy steel welds over the last two to three decades. The microstructure of weld metals and heat affected zones (HAZs) is known to be refined by different inclusions, which may act as nucleation sites for intragranular acicular ferrite and/or to pin austenite grains thereby preventing grain growth. In the present paper, the nature of acicular ferrite and the kinetics of intragranular ferrite transformations in both weld metals and the HAZ of steels are rationalised along with nucleation mechanisms. Acicular ferrite development is considered in terms of competitive nucleation and growth reactions at austenite grain boundary and intragranular inclusion nucleation sites. It is shown that compared to weld metals, it is difficult to shift the balance of ferrite nucleation from the austenite grain boundaries to the intragranular regions in the HAZ of particle dispersed steels because inclusion de...

Overview Inclusion assisted microstructure control in C–Mn and low alloy steel welds

Low carbon–manganese wrought steels with three different microstructures were prepared by different thermomechanical treatments without changing the chemical composition and their mechanical properties were compared. The microstructure of fine interlocking acicular ferrite nucleated intragranularly at Ti2O3 inclusions shows higher strength and good toughness at low temperatures than those of coarser polygonal ferrite/pearlite and aligned ferrite with second phase.

Inoculated acicular ferrite microstructure and mechanical properties

Mn absorption characteristics of Ti2O3 inclusions in low carbon steels

The influences of Si and Al on the microstructure of Ti-containing C-Mn steels have been studied. Although Si addition up to 1 wt pct did not change the crystal structure of dominant Ti2O3 particles dispersed in these steels, the formation of an acicular ferrite microstructure was suppressed at high Si contents. It has been found that the Mn content dissolved in Ti2O3 particles decreases with increasing Si content. This seems to weaken the formation of Mn-depleted zones (MDZs) around Ti2O3 particles, which act as a dominant driving force for the intragranular nucleation of acicular ferrite. It has also been found that the addition of a small amount of Al greatly reduces the formation of acicular ferrite. This is attributed to the change of dominant oxide particles from Ti2O3, which serves as an effective nucleant for acicular ferrite, to inert Al2O3. The result of the present study implies that it is advantageous to keep Si and Al contents in Ti-containing steels as low as possible when well-developed microstructures of acicular ferrite are desired.

Effects of Si and Al on acicular ferrite formation in C-Mn steel

The formation of non-metallic inclusions with variations in various oxygen, nitrogen and titanium contents in low carbon steels has been studied. The relationship between the nature of non-metallic inclusions and the formation of acicular ferrite has been also investigated. It is found that the non-metallic inclusions observed in this study are mainly consist of Ti 2 O 3 and TiN together with a small amount of other complex oxides containing Mn and Si. It has been confirmed that Ti 2 O 3 and TiN play an important role in formation of acicular ferrite as a nucleation site and an austenite grain refiner, respectively. But it is not clear yet whether TiN acts as a nucleation site or not. It is also found that the volume fraction of inclusions is more effective than the type or size distribution of the non-metallic inclusions on the formation of acicular ferrite.

Non-Metallic Inclusions and Acicular Ferrite in Low Carbon Steel

The influence of hot deformation on the formation of acicular ferrite in C-Mn wrought steels containing Ti 2 O 3 inclusions has been investigated. A significant amount of acicular ferrite is formed even after hot deformation. From this, it seems that the ability of Ti 2 O 3 inclusions to induce the nucleation of intragranular ferrite remains valid within recrystallized austenite grains after deformation. However, the fractions of acicular ferrite are lower than those in the steels without hot deformation. A high fraction of acicular ferrite is obtained after hot deformation when Mn level is high, the formation of intergranular ferrite such as allotriomorphic ferrite and upper bainite being pronouncedly suppressed. Deformation at relatively low temperatures discourages the formation of acicular ferrite while encouraging intergranular ferrite by reducing prior austenite grain size and consequently by increasing the area of the austenite grain boundaries.

https://www.jstage.jst.go.jp/article/isijinternational1989/40/8/40_8_819/_pdf

Jung-Soo Byun

Papers

Inoculated acicular ferrite microstructure and mechanical properties

Mn absorption characteristics of Ti2O3 inclusions in low carbon steels

Effects of Si and Al on acicular ferrite formation in C-Mn steel

Non-Metallic Inclusions and Acicular Ferrite in Low Carbon Steel

Hot Deformation and Acicular Ferrite Microstructure in C–Mn Steel Containing Ti2O3 Inclusions