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

Microstructure-mechanical properties relationships for quenching and partitioning (Q&P) processed steel

Nucleation of intragranular ferrite at Ti2O3 particle in low carbon steel

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

High-strength steel of the 490-MPa class strength for offshore structures was developed by making the most of a TiN-MnS complex precipitate effective that is over a wide welding heat input range. At the same time, its HAZ toughness improvement mechanism was clarified. The TiN-MnS system provided superior HAZ toughness with a medium heat input in -60°C Charpy test and -50°C CTOD test and with a high heat input in -60°C Charpy test. This is probably attributable to the TiN compound that pins austenite grains and to the TiN-MnS complex precipitate that produces fine intragranular ferrite (IGF) and reduce the fracture facet unit after welding. IGF may be formed either by increase in driving energy for nucleation in the region where manganese is depleted by the MnS precipitation or decrease in interfacial energy with nucleation by the crystal coherency of TiN and ferrite. The presence of manganese-depleted zone is an indispensable mechanism and is complemented by the crystal coherency of TiN and ferrite.

https://www.jstage.jst.go.jp/article/isijinternational1989/34/10/34_10_829/_pdf

Improvement in HAZ Toughness of Steel by TiN-MnS Addition

Improved Toughness of Weld Fussion Zone by Fine TiN Particles and Development of a Steel for Large Heat Input Welding

A dispersion of precipitates introduces a threshold stress which opposes deformation. The effect of precipitate shape on dispersion strengthening at elevated temperature has been examined. Three kinds of Mod. 9Cr-1Mo steels without Nb(C, N), with spherical Nb(C, N) and with complex Nb(C, N)+V(N, C) "V-wing" were designed to have the same microstructures except for the shape of precipitates. The threshold stresses were measured by tensile and creep tests at 600°C. Two sorts of effects have been observed. The first is that complex precipitates restrain dislocation from climbing even in a condition in which dislocation climbs over a spherical precipitate. Mobile dislocations are considerably trapped at the concavity of the V-wings during deformation. The second effect is that the probability of dislocation trapping at V-wing is larger than that at spherical Nb(C, N). These trapping effects play important roles in the strenghening effect of precipitate.

Effects of Precipitate Shape on High Temperature Strength of Modified 9Cr-1Mo Steels.

Flow stress of Si-Mn steel and Nb added steels measured under the usual hot deformation conditions for the rolling of steel plates was formulated by taking strain hardening and dynamic recovery into consideration. The functions derived include variables of deformation conditions such as deformation temperature, strain and strain rate and metallurgical factors such as austenite grain size and Nb concentration. The calculated stress-strain relations and the stress-dislocation density relations show close correlation with experimental data. The increase in flow stress by Nb addition is due to the additional effects of the increase in strain hardening rate and the decrease in dynamic recovery rate leading to a high level of dislocation density just after deformation.

Formulation of flow stress of Nb added steels by considering work-hardening and dynamic recovery

Decrease in dislocation density (ρ) in deformed austenite of Si-Mn steel and Nb added steels due to static recovery and recrystallization has been investigated. By comparing the flow stress measured at the second pass of a double compression test and the fraction of recrystallized austenite of the specimen quenched at a certain time after deformation, the change in ρ due to static recovery has been separated from the total change of ρ. The change in ρ due to static recovery has been formulated as functions of the concentration of Nb (Nb%), deformation conditions and time after deformation. The recovery process is mainly controlled by the climbing of dislocations in the case of higher deformation and holding temperature (DT) and lower Nb%, while it is mainly controlled by the annihilation of dislocations with opposite Burgers vectors in the case of lower DT and higher Nb%. Recovery is retarded by an increase in austenite grain size, a decrease in DT and an increase in Nb%.

Kentaro Okamoto

Papers

Improvement in HAZ Toughness of Steel by TiN-MnS Addition

Improved Toughness of Weld Fussion Zone by Fine TiN Particles and Development of a Steel for Large Heat Input Welding

Effects of Precipitate Shape on High Temperature Strength of Modified 9Cr-1Mo Steels.

Formulation of flow stress of Nb added steels by considering work-hardening and dynamic recovery

Formulation of the decrease in dislocation density of deformed austenite due to static recovery and recrystallization