Grain refinement in steels through thermomechanical processing

The development of ultrafine grained microstructures in steels has received considerable attention in recent times. In many cases the aim is to produce high strength structural steels with minimal alloying. It is well established that for an equiaxed ferrite with a uniform dispersion of second phase, both the strength and toughness will be markedly improved if the grain size can be reduced to 1-2 μm, from the typical range of 5-10 μm. Means of achieving this through dynamic strain induced transformation are examined here, following a brief overview of some of the key issues encountered when attempting to refine the austenite in existing mill configurations. A number of deformation microstructure maps are developed to aid the discussion.

Formation of ultra-fine ferrite in hot rolled strip: potential mechanisms for grain refinement

A novel single-pass hot strip rolling process has been developed in which ultra-fine (<2 μm) ferrite grains form at the surface of hot rolled strip in two low carbon steels with average austenite grain sizes above 200 μm. Two experiments were performed on strip that had been re-heated to 1250°C for 300 s and air-cooled to the rolling temperatures. The first involved hot rolling a sample of 0.09 wt.%C–1.68Mn–0.22Si–0.27Mo steel (steel A) at 800°C, which was just above the Ar3 of this sample, while the second involved hot rolling a sample of 0.11C–1.68Mn–0.22Si steel (steel B) at 675°C, which is just below the Ar3 temperature of the sample. After air cooling, the surface regions of strip of both steel A and B consisted of ultra-fine ferrite grains which had formed within the large austenite grains, while the central regions consisted of a bainitic microstructure. In the case of steel B, a network of allotriomorphic ferrite delineated the prior-austenite grain boundaries throughout the strip cross-section. Based on results from optical microscopy and scanning/transmission electron microscopy, as well as bulk X-ray texture analysis and microtextural analysis using Electron Back-Scattered Diffraction (EBSD), it is shown that the ultra-fine ferrite most likely forms by a process of rapid intragranular nucleation during, or immediately after, deformation. This process of inducing intragranular nucleation of ferrite by deformation is referred to as strain-induced transformation.

Effect of solidification grain refinement on the development of wrought Mg alloys

A recent trial investigated the effect of solidification grain refinement of billet on the grain refinement and properties of alloy ZM20. It was found that even at levels of 0.4Mn, significant grain refinement could be obtained when 0.7Zr was added. At 0.2Mn grain sizes as low as 60μm were
obtained. Billets of Mg-2Zn-0.2Mn with four different grain sizes, due to different Zr and cooling rates were then cast via vertical direct chill casting and extruded conventionally. Benefits of grain refinement of the billet on extrusion were found to be a slight increase in the size of the operating
window, and a reduction of the grain size in the extrudate. However, the effect of the reduction in extrudate grain size due to refinement of the billet was small compared with the amount of grain refinement obtained due to recrystallisation on extrusion.

Grain refinement through controlled thermomechanical processing

The major challenge for thermomechanical processing is to extend grain refinement towards lower average grain sizes. However, there is also a need to provide a better understanding of the mechanisms through which the refinement processes proceed. Many recent proposals for advanced thermomechanical processing rely on the dynamic strain induced transformation (DSIT), and the dynamic recrystallisation of austenite as the main refinement mechanisms. These mechanisms are still not fully understood and their clarification can be expected to lead to even greater levels of refinement. The current review examines the roles of ferrite recrystallisation , DSIT and initial austenite grain size. It is shown that although the ferrite recrystallisation mechanism in DSIT has certain similarities with the well known continuous dynamic recrystallisation (CDRX), it is significantly affected by the transformed ferrite grain size. Also, reducing the initial austenite grain size increases the recrystallisation rate in the strain dependent as well as strain independent regions. These results show that the traditional concept of metadynamic recrystallisation is insufficient to explain the changes in grain size following deformation. An alternative explanation is presented.

Grain refinement of pure magnesium using rolled Zirmax® master alloy (MG-33.3ZR)

Owing to the limited solubility of zirconium in molten magnesium, almost all of the zirconium contained in the Zirmax® master alloy (Mg-33.3Zr) is present in the form of nearly pure zirconium particles. Of them, zirconium particle clusters and individual zirconium particles greater than 5

High performance mg alloy by grain refinement

Mg alloys are one of promising eco-materials. The present paper describes the importance of grain refinement to develop high performance Mg alloys. The fine-grained Mg alloys exhibit not only a good combination of high strength and high ductility at room temperature, but also high formability (superplasticity) at elevated temperatures.

Grain refinement of an extruded Mg alloy via na microalloying

The effect of 0.3 wt pct Na on the microstructure of extruded alloy Mg-2Sn-1Zn is examined. We report that Na stabilizes the Mg2Sn phase, resulting in its precipitation during extrusion under conditions where a solid solution is otherwise expected. This effect appears to be thermodynamic in nature and is different from the kinetic enhancement of low- temperature aging reported by Mendis et al. [Phil. Mag. Letters, 86 (2006), 443]. The precipitates of the current study enable useful refinement of the grain size.

Grain refinement in titanium alloys through thermomechanical processing

Qi developed a novel thermomechanical processing route for the grain refinement of titanium alloys. This leads to a well-balanced superior mechanical property, which is vital for modern air transport. The outcomes of this project are prospective to enhance titanium application and the long-term viability of Australian resources and manufacturing industries.

β-Ti grain refinement via α-Precipitation

The present work explores the impact of α precipitates on β recrystallization following hot deformation of Ti-5Al-5Mo-5V-3Cr with grains larger than 1 mm. A single hot rolling pass of 36 pct reduction was conducted on an aged microstructure containing α precipitates at a temperature well below the β transus temperature. After annealing, a uniformly recrystallized structure with a grain size of ~100 µm is formed. The prior β grain boundaries can be readily identified and it is seen that the primary β grains have been replaced by grains displaying a spread of correlated misorientation angles extending up to the highest allowable values. The annealing comprises two stages. The first stage involves normal β subgrain growth limited by the Zener pinning force of the unstable α precipitates. The second stage corresponds to the onset of β recrystallization at the point where the Zener pinning force drops due to dissolution of the α precipitates. This leads to a uniform distribution of site saturated recrystallization nuclei.

Texture evolution and grain refinement of ultrafine-grained copper during micro-extrusion

Samples of oxygen-free high conductivity (OFHC) coarse-grained (CG) and ultrafine-grained (UFG) copper were micro-extruded to an equivalent strain of 2.8 in one pass at room temperature. Samples of the OFHC copper were annealed at 650C for 2 h to produce CG copper. Some samples were subsequently processed by equal channel angular pressing of eight passes, route Bc, at room temperature to produce the UFG material. Crystallographic texture and misorientation distributions were obtained locally from EBSD mappings at different radial positions after micro-extrusion. To model the strain path during micro-extrusion, the analytic flow line model of Altan etal. [J Mater. Process. Tech. 33 (1992) p.263] was used and also validated by finite element calculations. Modelling was carried out using the viscoplastic self-consistent (VPSC) model and a recently developed grain refinement model. The results showed large texture variations along the cross-section of the extruded sample for both UFG and CG copper. These cyclic drawing textures in UFG copper were simulated in good agreement with experiments using the presented modelling framework.

Grain size in Mg alloys: recrystallization and mechanical consequences

The present paper examines the development of grain size during the recrystallization of magnesium alloys and the influence the grain size has on the mechanical response. In magnesium alloys grain refinement improves the strength-ductility balance. This simultaneous increase in both strength and ductility is ascribed to the impact the grain size has on deformation twinning. The mechanisms by which the grain size is established during hot working are shown to be conventional dynamic recrystallization followed by post-dynamic recrystallization. The role of alloying additionon both of these reactions is briefly considered.

Study of the effect of grain size on the dynamic mechanical properties of microalloyed steels

In the present paper the effect of grain refinement on the dynamic response of ultra fine-grained (UFG) structures for C–Mn and HSLA steels is investigated. A physically based flow stress model (Khan-Huang-Liang, KHL) was used to predict the mechanical response of steel structures over a wide range of strain rates and grain sizes. However, the comparison was restricted to the bcc ferrite structures. In previous work [K. Muszka, P.D. Hodgson, J. Majta, A physical based modeling approach for the dynamic behavior of ultra fine-grained structures, J. Mater. Process. Technol. 177 (2006) 456–460] it was shown that the KHL model has better accuracy for structures with a higher level of refinement (below 1 μm) compared to other flow stress models (e.g. Zerrili-Armstrong model). In the present paper, simulation results using the KHL model were compared with experiments. To provide a wide range of the experimental data, a complex thermomechanical processing was applied. The mechanical behavior of the steels was examined utilizing quasi-static tension and dynamic compression tests. The application of the different deformation histories enabled to obtain complex microstructure evolution that was reflected in the level of ferrite refinement.