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The annealing behaviour, including studies of recrystallisation kinetics and development of crystallographic texture, of two low carbon steels after different cold rolling reductions have been investigated using Optical Microscopy (OM), Electron Back-Scatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). The primary recrystallisation behaviour of 20, 50, 70 and 90% cold rolled Interstitial Free (IF) and High Strength Low Alloy (HSLA) steels was studied. The HSLA was initially processed to give a volume fraction of about 0.2 of fine pearlite colonies, which acted as mechanically hard particles. The presence of such particles on the HSLA steel significantly reduced the temperature needed for recrystallisation by enhancing the recrystallisation and acting as nucleation sites by the Particle Stimulated Nucleation (PSN) mechanism. The inhomogeneous deformation and the local orientation changes introduced in the neighbourhood of the carbide particles (i.e. the particle deformation zone) were observed using TEM and selected area electron diffraction. The JMAK model was used to analyse the recrystallisation kinetics of the two steels. The experimental data plotted according to the JMAK model could be represented by straight lines with a JMAK exponent n falling in the range from 1.4 to 2.0. The development of crystallographic textures after cold rolling reductions and subsequent recrystallisation has been investigated. The texture development is shown to be largely dependent on the rolling reduction. With increasing rolling reduction, the annealing texture show gradual intensification of α- and γ-fibre components. However, the intensity of both fibres is weaker in the HSLA steel. Despite the dominance of PSN in HSLA steel, the recrystallisation textures were similar to that of the rolling textures with weaker intensity of both fibres. To help clarify the reasons for that, in-situ EBSD experiments of recrystallising HSLA steel deformed to 50% and 70% have been carried out. It shows that the formation of the nucleus seems to occur within the deformation zones in regions away from the particle surface leading to recrystallisation textures similar to that of the rolling textures (i.e. both α- and γ-fibre exist). The validity of this assumption has been confirmed by the use of Monte Carlomodelling. This model was used to simulate, in general way, and study the phenomenon of PSN during the recrystallisation process. The simulation shows the discontinuous evolution of the subgrains in the deformation zone to form recrystallisation nuclei around the particle. It shows also the subsequent growth of these nuclei to consume the matrix region around the particle. The simulation results are shown to match with the experimentally observed features of the recrystallisation phenomena in low carbon steel containing coarse cementite particles.

In this study a new method is introduced to produce bimodal grain structures in low carbon steels. This method is based on cold rolling of dual phase structures and appropriate annealing treatments. The difference in the recrystallization behaviour of ferrite and martensite yields a heterogeneous microstructure with a distribution of coarse and fine grains. These types of microstructures are of interest for optimizing the balance of strength and uniform elongation in ultra-fine grained low carbon steels.

Casting of test castings from low – carbon steel with different deoxidation degrees. Oxygen activity measurement in castings – from the temperatures over liquidus to solidification of metal. Analysis of chemical composition of inclusions. Calculation of thermodynamic equilibrium between oxygen activity and aluminium, silicon and manganese activities in unalloyed steel. Estimation of conditions for the course of carbon reactions in studied casting.

Abstract In this thesis, methods to process ultrafine ferritic (UFF) structures in steels, i.e. grain sizes below about 3 μm have been investigated. It is shown here, in accordance with the results in the literature, that a steel with a UFF grain size can be obtained by two methods, more or less convenient to mass production: deformation-induced ferrite transformation from fine-grained austenite (the DIF route) and the static recrystallization of various heavily cold-worked initial microstructures (the SRF/SRM route). In the present work, the influencing factors in the processing of UFF structure in the DIF route have been systematically studied in four low-carbon steels: one C-Mn steel and Nb, Nb-Ti and Nb-high Ti microalloyed steels. A high strain, a low deformation temperature close to Ar3 and a fine prior austenite grain size are beneficial to promote the formation of UFF grains. Especially by using complex pretreatments to refine the prior austenite grain size, cold rolling, repeating the low-temperature reheating cycle or using martensitic initial microstructure, a UFF grain size can be obtained in these steels at the strain of 1.2 (70% reduction) at 780 °C. By controlling the cooling rate, the type of the second phase can be adjusted. When using the static recrystallization route, it was found that UFF is difficult to obtain from a single-phase ferrite, but it is relatively readily obtained from deformed pearlite, bainite or martensite, especially in high-carbon steels with 0.3–0.8%C. In deformed pearlite, the cementite lamellae fragmented and spheroidised in the course of heavy deformation can provide numerous nucleation sites by the particle stimulated nucleation mechanism and retard the subgrain and recrystallized grain growth. Nucleation and retardation of grain growth are effective also in deformed bainite, martensite or high-carbon tempered martensite, as discussed in detail in the work. The thermal stability of UFF grained steels was tested and found to be generally excellent, but it varies depending on the processing method. The UFF structure obtained by the SRM route has a thermal stability somewhat weaker than that of the DIF route. For a given steel, UFF grains may show different grain growth modes, related to the dispersion of second phase particles. In the DIF structure, abnormal grain growth occurs at 700 °C after about 2.5 h, while in the SRM structure, normal grain growth takes place slowly at 600 °C. Carbides on the grain boundaries seem to play an important role in inhibiting grain coarsening. No coarse-grained zone was formed at the HAZ of electron beam or laser welded seams, as performed at low heat inputs (up to 1.5 kJ/cm) on thin strips. The hardness even increased from the base metal towards the HAZ and the weld metal in all seams as an indication that they were hardened during the rapid cooling.