Ultrafine ferrite formation in steels through thermomechanical processing

The main aim of this study was to investigate the critical conditions for the formation of ultrafine grain structures using hot torsion and wedge rolling techniques. In addition, the effect of thermomechanical parameters and steel composition on the critical conditions for ultrafine grain structure formation has been systematically evaluated.

Low strain processing of LC and ULC steels

Temper rolling and tension levelling are commonly used to manufacture flat rolled steel. Both processes lengthen the steel at strains up to 3% by applying a load and stretching the strip. By latering the balance between the load and the tension the formability of the low carbon and ultra low carbon steel may be optimised.

Fluidised bed nitrocarburising of tool steels

The work examines how ferritic nitrocarburising, a common heat treatment process for steels, can be adapted to the fluidised bed furnace. The effect of fluidising powder contamination on repeatability, composition of the reactive gas mixture, the development of nitrocarburised tool steel microstructure and resulting wear properties were investigated.

Experimental analysis of friction in forming automotive coated steels

The thesis addressed and answered a number of key issues in the experimental analysis of friction in sheet forming operations. Conventional friction theories were linked with the properties of sheet coatings and the process geometries. Newly derived mathematical models extended the analysis of friction in sheet metal forming applications.

Springback behaviour of TRIP steels in sheet metal forming

Examines the methods for numerical modelling of the springback effect in TRansformation Induced Plasticity (TRIP) steels under conditions of various bending processes. It represents a largely unexplored part of the TRIP steel literature and therefore makes a valuable contribution toward a practical approach to predicting springback in TRIP steels.

Advanced modelling of forming high strength steels

The thesis presents a novel approach in the multiscale modelling of Advanced High Strength Steels for prediction of the microstructural effects in forming processes. The results are compared with that of experiments and finite element method. The method is proved to be suitable for complexities in the multiphase AHSS.

Variant selection during the c-to-ab phase transformation in hot-rolled bainitic TRIP-aided steels

The variant selection phenomenon during the austenite to bainite phase transformation in hot-rolled TRIP-aided steels was quantitatively characterized at the level of individual austenite grains. The reconstruction of the electron backscatter diffraction maps provided evidence that bainite grows by packets of laths sharing a common {1 1 1}_y plane in the austenite. The affect of hot deformation is to reduce the number of packets that form. It is suggested that slip activity is important in understanding this effect.

Microstructure evolution modeling during and after deformation in 304 austenitic stainless steel through cellular automaton approach

A 2D cellular automation approach was used to simulate microstructure evolution during and after hot deformation. Initial properties of the microstructure and dislocation density were used as input data to the cellular automation model. The flow curve and final grain size were the output data for the dynamic recrystallization simulation, and softening kinetics curves were the output data of static and metadynamic recrystallization simulations. The model proposed in this work considered the effect of thermomechanical parameters (e.g., temperature and strain rate) on the nucleation and growth kinetics during dynamic recrystallization. The dynamic recrystallized microstructures at different strains, temperatures, and strain rates were used as input data for static and metadynamic recrystallization simulations. It was shown that the cellular automation approach can model the final microstructure and flow curve successfully in dynamic recrystallization conditions. The postdeformation simulation results showed that the time for 50% recrystallization decreases with increasing strain for a given initial grain size and that dynamic recrystallization slows the postdeformation recrystallization kinetics compared to a model without dynamic recrystallization.

Formability and forming of advanced steels

The thesis identified how advanced high strength steels perform compared to conventional steels in terms of weight reduction and crash performance for automotive bodies. The novel production method of low pressure tube hydroforming was applied to form these advanced steels to reduce the press tonnage and fluid pressure compared to the conventional high pressure process. In addition analytical models were developed to predict the force and pressure in the low pressure process.

Characterization of the bake-hardening behavior of transformation induced plasticity and dual-phase steels using advanced analytical techniques

The bake-hardening (BH) behavior of TRansformation Induced Plasticity (TRIP) and Dual-Phase (DP) steels after intercritical annealing (IA) has been studied using transmission electron microscopy, X-ray diffraction and three dimensional atom probe tomography. It was found for the DP steel that carbon can segregate to dislocations in the ferrite plastic deformation zones where there is a high dislocation density around the "asquenched" martensite. The carbon pinning of these dislocations, in turn, increases the yield strength after aging. It was shown that bake-hardening also leads to rearrangement of carbon in the martensite leading to the formation of rod-like low temperature carbides in the DP steel. Segregation of carbon to microtwins in retained austenite of the TRIP steel was also evident. These factors, in combination with the dislocation rearrangement in ferrite through the formation of cells and microbands in the TRIP steel after pre-straining, lead to the different bake-hardening responses of the two steels.

Effect of manufacturing process on the final properties of advanced high strength steels for automotive applications

The performance of multiphase steels with high strength and improved toughness or ductility, such as intercritically annealed dual-phase (DP) and transformation-induced plasticity (TRIP) steels, is of key importance to the automotive industry. In this work we have considered the entire manufacturing process and the effects of this on the final product performance. These steels are formed to produce the required final shape and then the car is paint baked. In this work we also consider the effect of cold working and bake hardening on the fatigue life of the components.

Nano-scale analysis of nano-bainite formed in advanced high strength steels

The effect of composition and processing schedule on the microstructure of C-Mn-Si-Mo-(Al)-(Nb) steels containing nano-bainite was studied using transmission electron microscopy (TEM) and atom probe tomography (APT). The major phase formed in all steels was nano-bainite. However, the steels with lower carbon and alloying addition content subjected to TMP had better mechanical properties than high alloyed steel after isothermal treatment. The presence of ferrite in the microstructure can improve not only ductility but lead to the formation of retained austenite with optimum chemical stability.

Surface alloying of metals using fluid bed reactor

The use of fluidised bed reactors for surface alloying is reviewed. Research at Deakin has includes the use of chemical vapour deposition to form chromium rich layers on ferrous substrates, including stainless and tool steel grades. These layers can be modified to carbide or nitride if required by the end application. The deposition of aluminium and silicon has also been successfully achieved.