Refinement of microstructure in steels through thermomechanical processing

The refinement of microstructure is the most generally accepted approach to simultaneously improve the strength and toughness in steels. In the current study, the role of dynamic/static phase transformation on the ferrite grain refinement was investigated using different thermomechanical processing routes. A Ni-30Fe austenitic model alloy was also used to investigate the substructure character formed during deformation. It was revealed that the microstructure of steel could further be refined to the nanoscale through both the control of processing route and steel composition design.

Effect of transformation mechanism (static or dynamic) on final ferrite grain size

A simple series of test was developed to highlight and compare the difference between the static strain induced transformation (SSIT) and the dynamic strain induced transformation (DSIT) mechanism in grain refinement and also to investigate the origin of the difference between the two mechanisms. The results showed that while the SSIT sets up a two-dimensional impingement among the ferrite grains, it cannot avoid their coarsening (normal growth). However, the DSIT forms a group of grains with a three-dimensional impingement which does not coarsen and maintains their fine size throughout the transformation, thereby, reduces the final average grain size.

Effect of grain size on corrosion of high purity aluminium

A complete understanding of how grain refinement, grain size, and processing affect the corrosion resistance of different alloys has not yet been fully developed. Determining a definitive 'grain size-corrosion resistance' relationship, if one exists, is inherently complex as the processing needed to achieve grain refinement also imparts other changes to the microstructure (such as texture, internal stress, and impurity segregation). This work evaluates how variation in grain size and processing impact the corrosion resistance of high purity aluminium. Aluminium samples with a range of grain sizes, from ∼100 μm to ∼2000 μm, were produced using different processing routes, including cold rolling, cryo rolling, equal channel angular pressing, and surface mechanical attrition treatment. Evaluation of all the samples studied revealed a tendency for corrosion rate to decrease as grain size decreases. This suggests that a Hall-Petch type relationship may exist for corrosion rate and grain size. This phenomenon, discussed in the context of grain refinement and processing, reveals several interesting and fundamental relationships.

Ultrafine grain formation in a Ti-6Al-4V alloy by thermomechanical processing of a martensitic microstructure

In the current study, ultrafine equiaxed grains with a size of 150 to 800 nm were successfully produced in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. This was achieved through a novel mechanism of grain refinement consisting of several concurrent processes. This involves the development of substructure in the lath interiors at an early stage of deformation, which progressed into small high-angle segments with increasing strain. Consequently, the microstructure was gradually transformed to an equiaxed ultrafine grained structure, mostly surrounded by high-angle grain boundaries, through continuous dynamic recrystallization. Simultaneously, the supersaturated martensite was decomposed during deformation, leading to the progressive formation of beta phase, mainly nucleated on the intervariant lath boundaries.

Hot working microstructure map for magnesium AZ31

A thermomechanical processing (TMP) structure map is proposed that plots the critical strains required for dynamic recrystallization along with the grain sizes that result. These maps are useful in identifying the limits to grain refinement and designing hot working processes. They are readily constructed for well studied alloys such as plain carbon steel. In light of the recent interest in the hot working of magnesium, initial steps are taken here to construct a TMP structure map for the most common wrought magnesium alloy, AZ31. The completion of dynamic recrystallization is estimated using a geometrical approach and a twinning region is identified.

Microstructural evolution of ultrafine grained structure in plain carbon steels through single pass rolling

In the present study, the effect of nominal equivalent strain (between 0 and 1.2), deformation temperature (790– 750°C) and carbon content (0.06 – 0.35%C) was investigated on ferrite grain refinement through dynamic strain induced transformation (DSIT) in plain carbon steels in single pass rolling. The microstructural evolution of the transformation of austenite to ferrite has been evaluated through the thickness of the strip. The results showed a number of important microstructural features as a function of strain, which could be classified into three regions; no DSIT region, DSIT region, and ultrafine ferrite (UFF) grain region. Hence, two critical strains; dynamic strain induced transformation (εC, DSIT) and ultrafine ferrite formation (εC, UFF) were determined. These strains were increased significantly with an increase in carbon content. The critical strain for UFF formation reduced with decrease in deformation temperature. The UFF microstructure consisted of ultrafine, equiaxed ferrite grains (<2 μm) with very fine cementite particles. In the centre of the rolled strip, there was a conventional ferrite– pearlite microstructure, although ferrite grain refinement and the volume fraction of ferrite increased with increase in the nominal equivalent strain.

The static, dynamic and metadynamic recrystallisation of a medium carbon steel

Grain refinement during and after hot isothermal deformation of a medium carbon steel has been investigated. The average austenite grain size decreased with an increase in strain for the hot deformed and recrystallised material, with refinement extending beyond the strain for the peak stress. A window of strain that corresponds to transition from classical static to metadynamic recrystallisation was observed in respect to the recrystallised material. Within this post-dynamic transition window the strain at which strain independent softening occurs was different for different volume fractions of the recrystallised material. This led to a new terminology corresponding to initiation of strain independent softening. For the alloy of this study, strain independent softening for the start of post-deformation recrystallisation occurred near the strain to the peak stress. The strain corresponding to complete metadynamic recrystallisation, which was defined as when all levels of recrystallisation were strain independent, was much greater than the strain for the peak stress.

Formation of ultrafine grained structure in plain carbon steels through thermomechanical processing

In the present study, wedge-shaped samples were used to determine the effect of nominal equivalent strain (between 0 and 1.2) and carbon content (0.06--0.35%C) on ferrite grain refinement through dynamic strain-induced transformation (DSIT) in plain carbon steels using single-pass rolling. The microstructural evolution of the transformation of austenite to ferrite has been evaluated through the thickness of the strip. The results showed a number of important microstructural features as a function of strain which could be classified into three regions; no DSIT region, DSIT region and the ultrafine ferrite (UFF) grain region. Also, the extent of these regions was strongly influenced by the carbon content. The UFF microstructure consisted of ultrafine, equiaxed ferrite grains (<2 μ$m) with very fine cementite particles. In the centre of the rolled strip, there was a conventional ferrite-pearlite microstructure, although ferrite grain refinement and the volume fraction of ferrite increased with an increase in the nominal equivalent strain.

The static and metadynamic recrystallization behaviour of an X60 Nb Microalloyed Steel

A rapid method has been developed to determine recrystallization kinetics of Nb microalloyed steels by interrupted hot torsion test. The softening behaviour was achieved as a function of different processing parameters. The method clearly identified three regions, where the strain dependency of the recrystallization rate varied. Firstly, at large strains the rate of recrystallization was not a function of strain; this is generally ascribed to metadynamic recrystallization. At lower strains the time to 50% recrystallization showed a power low relationship with strain, characteristic of static recrystallization. A further break point exists on the time for 50% softening curve when strain induced precipitation occurs in the material. The onset of strain induced precipitation was at strains below the strain to the peak stress at temperatures below 900°C. The experimental results were used to estimate the time for 50% softening and to anticipate the onset of the strain induced precipitation for the alloy of this study. Grain refinement of the recrystallized austenite continued to strains significantly beyond the peak stress and beyond the static to metadynamic recrystallization rate transition.

Static and metadynamic recrystallization of interstitial free steels during hot deformation

A rapid method was used to identify kinetics of the recrystallization for two IF (Interstitial Free) steels which have different phosphorous and boron contents. The static and metadynamic softening behaviour of the materials for a range of strain rates and temperatures were quantified. The critical strain for initiation of strain independent softening was estimated for the IF steels in respect to the time for 50 percent softening after deformation. The results showed that the strain for the initiation of strain independent softening (often referred to as metadynamic recrystallization) varies with the Zener Hollomon parameter. Classic static recrystallization was observed at strains below the strain independent softening for all processing conditions and the strain rate had a strong effect on the time for strain independent softening. Results also revealed that static and metadynamic recrystallization was delayed owing to the phosphorous and boron alloying elements. Hence, the large strain at above no-recrystallization temperature may be required for the early stage of Finishing Stands Unit (FSU) in hot strip rolling mills to initiate austenite grain refinement of phosphorous and boron added IF steels.

Superplasticity and cavitation of recycled AZ31 magnesium alloy fabricated by solid recycling process

Superplastic behaviour of Mg-alloy AZ31 was investigated to clarify the possibility of its use for superplastic forming (SPF) and to accurately evaluate material characteristics under a biaxial stress by utilizing a multi-dome test. The material characteristics were evaluated under three different superplastic temperatures , 643, 673, and 703 K in order to determine the most suitable superplastic temperature. Finite Element Method (FEM) simulation of rectangular pan forming was carried out to predict the formability of the material into a complex shape. The superplastic material properties are used for the simulation of a rectangular pan. Finally, the simulation results are compared with the experimental results to determine the accuracy of the superplastic material characteristics. The experimental results revealed that the m values are greater than 0.3 under the three superplastic temperatures, which is indicative of superplasticity. The optimum superplastic temperature is 673 K, at which a maximum m value and no grain growth were observed. The results of the FEM simulation revealed that certain localized thinning occurred at the die entrance of the deformed rectangular pan due to the insufficient ductility of the material. The simulation results also showed that the optimum superplastic temperature of AZ31 is 673 K.

Precipitate characterisation of an advanced high-strength low-alloy (HSLA) steel using atom probe tomography

Increased fuel economy, combined with the need for the improved safety has generated the development of new hot-rolled high-strength low-alloy (HSLA) and multiphase steels such as dual-phase or transformation-induced plasticity steels with improved ductility without sacrificing strength and crash resistance. However, the modern multiphase steels with good strength-ductility balance showed deteriorated stretch-flangeability due to the stress concentration region between the soft ferrite and hard martensite phases [1]. Ferritic, hot-rolled steels can provide good local elongation and, in turn, good stretch-flangeability [2]. However, conventional HSLA ferritic steels only have a tensile strength of not, vert, similar600 MPa, while steels for the automotive industry are now required to have a high tensile strength of not, vert, similar780 MPa, with excellent elongation and stretch-flangeability [1]. This level of strength and stretch-flangeability can only be achieved by precipitation hardening of the ferrite matrix with very fine precipitates and by ferrite grain refinement. It has been suggested that Mo [3] and Ti [4] should be added to form carbides and decrease the coiling temperature to 650 °C since only a low precipitation temperature can provide the precipitation refinement [4]. These particles appeared to be (Ti, Mo)C, with a cubic lattice and a parameter of 0.433 nm, and they were aligned in rows [4]. It was reported [4] that the formation of these very fine carbides led to an increase in strength of not, vert, similar300 MPa. However, the detailed analysis of these particles has not been performed to date due to their nanoscale size. The aim of this work was to carry out a detailed investigation using atom probe tomography (APT) of precipitates formed in hot-rolled low-carbon steel containing additions Ti and Mo.

The investigated low-carbon steel, containing Fe–0.1C–1.24Mn–0.03Si–0.11Cr–0.11Mo–0.09Ti–0.091Al at.%, was produced by hot rolling. The processing route has been described in detail elsewhere [5] European Patent Application, 1616970 A1, 18.01.2006.[5]. The microstructure was characterised by transmission electron microscopy (TEM) on a Philips CM 20, operated at 200 kV using thin foil and carbon replica techniques. Qualitative energy dispersive X-ray spectroscopy (EDXS) was used to analyse the chemical composition of particles. The atomic level of particle characterisation was performed at the University of Sydney using a local electrode atom probe [6]. APT was carried out using a pulse repetition rate of 200 kHz and a 20% pulse fraction on the sample with temperature of 80 K. The extent of solute-enriched regions (radius of gyration) and the local solute concentrations in these regions were estimated using the maximum separation envelope method with a grid spacing of 0.1 nm [7]. A maximum separation distance between the atoms of interest of dmax = 1 nm was used.

The microstructure of the steel consisted of two types of fine ferrite grains: (i) small recrystallised grains with an average grain size of 1.4 ± 0.2 μm; and (ii) grains with a high dislocation density (5.8 ± 1.4 × 1014 m−2) and an average grain size of 1.9 ± 0.1 μm in thickness and 2.7 ± 0.1 μm in length (Fig. 1a). Some grains with high dislocation density displayed an elongated shape with Widmanstätten side plates and also the formation of cells and subgrains (Fig. 1a). The volume fraction of recrystallised grains was 34 ± 8%.

Dynamic adjustment of ferrite grains during dynamic strain induced transformation

The dynamic adjustment of ferrite grains formed during 'dynamic strain induced transformation (DSIT)' is an important feature of this mechanism that has not been addressed previously. A novel experimental method was applied to follow the effect of deformation at different stages on ferrite formed initially through DSIT. It is shown that while the continuous dynamic recrystallisation (CDRX) appears to be an acceptable mechanism for re-refinement of coarser grain size (i.e. dα>2dDSIT), it cannot explain the steady state grain size for finer ferrite grains (i.e. dα<2dDSIT). Other potential mechanisms involved in this phenomenon are examined.

Modeling of the mechanical behavior of nanostructured HSLA steels

In the present paper the basic strengthening mechanisms operating in microstructures are discussed with respect to their application in submicron/nano materials. This analysis focuses on these strengthening mechanisms in bcc microstructures, where the effect of grain boundaries is very strong. An experimental study of the influence of the thermomechanical history on the microstructure and dislocation substructure was performed using two different grades of HSLA steels. As a result, a modified version of the Khan–Huang–Liang flow stress model (KHL) was developed and is discussed in the light of results from the present study. Comparison with experimental results showed significant diversity in the refinement and mechanical responses of each steel, due to different activity of strengthening mechanisms and microalloying elements in the microstructure evolution process. The effect of mechanical and microstructural inhomogeneity in severe plastic deformation (SPD) on the deformation induced grain refinement and mechanical properties was also considered.

A physical based modeling approach for the dynamic behavior of ultrafine grained structures

This paper discusses some experimental results on the influence of grain refinement on the final mechanical properties of IF and microalloyed steels designed for auto-body components. It shows also some modeling approaches to understanding the dynamic behavior of fine-rained materials. The Zerilli–Armstrong (Z–A) and Khan–Huang–Liang (KHL) models for studied steels were implemented into FEM code in order to simulate the dynamic compression tests with different strain rates.