Ilana Timokhina

Transformation behaviour in thermomechanically processed C-Mn-Si TRIP steels with and without Nb

Abstract Two 0.2wt.% C–1.55wt.% Mn–1.5wt.% Si steels with and without the addition of 0.039 wt.% Nb were studied by laboratory simulations of controlled thermomechanical processing in a quench deformation dilatometer. The microstructures were characterised using optical metallography, image analysis and scanning electron microscopy techniques. The effects of recrystallised and non-recrystallised austenite and discontinuous paths on the final microstructure were studied. The results have shown that the highest volume fraction of retained austenite is associated with a 400°C isothermal bainite transformation temperature and the presence of approximately 50% polygonal ferrite and acicular ferrite as a dominant second phase.

Constitutive Modeling of the Tensile Behavior of Al-TWIP Steel

High Mn steels demonstrate an exceptional combination of high strength and large ductility as a result of their high strain-hardening rate during deformation. The microstructure evolution and strain-hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uniaxial tension were examined. The purpose of this study was to determine the contribution of all the relevant deformation mechanisms—slip, twinning, and dynamic strain aging. Constitutive modeling was carried out based on the Kubin–Estrin model, in which the densities of mobile and forest dislocations are coupled to account for the interaction between the two dislocation populations during straining. These coupled dislocation densities were used to simulate the contribution of dynamic strain aging to the flow stress. The model was modified to include the effect of twinning. To ascertain the validity of the model, the microstructural evolution was characterized in detail by means of transmission electron microscopy and electron back-scatter diffraction.

Understanding the Behavior of Advanced High-Strength Steels Using Atom Probe Tomography

The key evidence for understanding the mechanical behavior of advanced high strength steels was provided by atom probe tomography (APT). Chemical overstabilization of retained austenite (RA) leading to the limited transformation-induced plasticity (TRIP) effect was deemed to be the main factor responsible for the low ductility of nanostructured bainitic steel. Appearance of the yield point on the stress-strain curve of prestrained and bake-hardened transformation-induced plasticity steel is due to the unlocking from weak carbon atmospheres of newly formed during prestraining dislocations.

Microstructure and mechanical properties of C-Si-Mn(-Nb) TRIP steels after simulated thermomechanical processing

Abstract Continuous and discontinuous cooling tests were performed using a quench deformation dilatometer to develop a comprehensive understanding of the structural and kinetic aspects of the bainite transformation in low carbon TRIP (transformation induced plasticity) steels as a function of thermomechanical processing and composition. Deformation in the unrecrystallised austenite region refined the ferrite grain size and increased the ferrite and bainite transformation temperatures for cooling rates from 10 to 90 K s-1. The influence of niobium on the transformation kinetics was also investigated. Niobium increases the ferrite start transformation temperature, refines the ferrite microstructure, and stimulates the formation of acicular ferrite. The effect of the bainite isothermal transformation temperature on the final microstructure of steels with and without a small addition of niobium was studied. Niobium promotes the formation of stable retained austenite, which influences the mechanical properties of TRIP steels. The optimum mechanical properties were obtained after isothermal holding at 400°C in the niobium steel containing the maximum volume fraction of retained austenite with acicular ferrite as the predominant second phase.

Three-dimensional atomic scale analysis of microstructures formed in high strength steels

Abstract The microstructures of steels consisting of ferrite (steel 1) and bainite (steel 2) were characterised using atom probe tomography. The microstructural features, such as formation of Nb–C clusters in steel 1 fine Fe–C particles, were observed.

Addressing Retained Austenite Stability in Advanced High Strength Steels

Advances in the development of new high strength steels have resulted in microstructures containing significant volume fractions of retained austenite. The transformation of retained austenite to martensite upon straining contributes towards improving the ductility. However, in order to gain from the above beneficial effect, the volume fraction, size, morphology and distribution of the retained austenite need to be controlled. In this regard, it is well known that carbon concentration in the retained austenite is responsible for its chemical stability, whereas its size and morphology determines its mechanical stability. Thus, to achieve the required mechanical properties, control of the processing parameters affecting the microstructure development is essential.

Microstructure and mechanical properties of thermomechanically processed C-Si-Mn steels

Abstract Two TRIP steels having high and low silicon contents have been subjected to simulated thermomechanical processing (rolling and coiling) in a quench dilatometer or a laboratory rolling mill. The dilatometer specimens showed a higher level of retained austenite than the rolled specimens, which had higher contents of martensite, and also of pearlite, reflecting the less rapid cooling in the larger rolling specimens. The best combination of strength and ductility was achieved after simulated coiling at 400–450°C, when the final microstructure of the high silicon steel consisted of ∼55–60%polygonal ferrite with small amounts of martensite/ retained austenite (∼4–5%) and carbide free bainitic phases. Martensite and retained austenite were not observed in the low silicon steel. Results from tensile and shear punch tests were found be in good agreement, confirming the viability of the micropunch method where insufficient material is available for full tensile tests (e.g. with dilatometer specimens). The scaling constant relating the tensile and punch test results is dependent on specimen composition and microstructure.

Local Laser Heat Treatment in Dual-Phase Steels

This research deals with processes leading to local strengthening effects in hot-rolled dual-phase (DP) steels. For this purpose, a method was investigated to achieve local strengthening, namely, local laser heat treatment (LHT). DP sheet steels were globally and homogenously deformed with different degrees of prestrains by cold rolling and subsequently locally heat treated by laser. Following this treatment with selected parameters, the microstructure of the surface and cross section of the heat-treated area as well as the mechanical properties were evaluated by light optical microscopy (LOM), scanning electron microscopy (SEM), as well as transmission electron microscopy (TEM), hardness measurement, and tensile testing. It can be stated that with partial heat treatment, local high strengthening can be produced. At lower heat treating temperatures, this effect could be attributed to bake hardening (BH). Increasing the prestrain as well as temperature results in improving the local properties. With increased heat treating temperature, the initial microstructure near the surface is affected. Partial strengthening of DP steels by laser can open up new fields of application for locally using the strengthening effect to only influence relevant areas of interest, thus providing the potential for saving energy and designed the component’s behavior.

Development of asymmetric rolling for the better control over structure and mechanical properties in IF steel

In the present study, the effects of kinematic and geometric asymmetries in rolling during multi-pass processing of IF steel are examined. The theoretical investigation by final element simulations and experimental investigations by means of electron-backscatter diffraction analysis and tensile tests suggest that asymmetric rolling increases the total imposed strain compared to symmetric rolling, and largely re-distributes the strain components due to additional shear. This enhances the intensity of grain refinement, strengthens and tilts crystallographic orientations, and increases mechanical strength. The effect is highest in the asymmetric rolling with differential roll diameters.

Understanding of the Bainite Transformation in a Nano-Structured Bainitic Steel

A 0.79C-1.5Si-1.98Mn-0.98Cr-0.24Mo-1.06Al-1.58Co (wt%) steel was isothermally heat treated at 200°C for 10 days to form a nano-scale bainitic microstructure consisting of nanobainitic ferrite laths with high dislocation density and retained austenite films. The crystallographic analysis using TEM and EBSD revealed that the bainitic ferrite laths are close to the Nishiyama-Wassermann orientation relationship with the parent austenite. There was only one type of packet identified in a given transformed austenite grain. Each packet consisted of two different blocks having variants with the same habit plane, but different crystallographic orientations. The presence of fine C-rich clusters and Fe-C carbides with a wide range of compositions in bainitic ferrite was revealed by Three-dimensional Atom Probe Tomography (APT). The high carbon content of bainitic ferrite compared to the para-equilibrium level of carbon in ferrite, absence of segregation of carbon to the austenite/bainitic ferrite interface and absence of partitioning of substitutional elements between the retained austenite and bainitic ferrite were also found using APT.