Deformation and Recrystallisation in Low Carbon Steels

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Abstract

The annealing behaviour, including studies of recrystallisation kinetics anddevelopment of crystallographic texture, of two low carbon steels after different coldrolling reductions have been investigated using Optical Microscopy (OM), ElectronBack-Scatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM).The primary recrystallisation behaviour of 20, 50, 70 and 90% cold rolled InterstitialFree (IF) and High Strength Low Alloy (HSLA) steels was studied. The HSLA wasinitially 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 theHSLA steel significantly reduced the temperature needed for recrystallisation byenhancing the recrystallisation and acting as nucleation sites by the ParticleStimulated Nucleation (PSN) mechanism. The inhomogeneous deformation and thelocal orientation changes introduced in the neighbourhood of the carbide particles(i.e. the particle deformation zone) were observed using TEM and selected areaelectron diffraction. The JMAK model was used to analyse the recrystallisationkinetics of the two steels. The experimental data plotted according to the JMAKmodel could be represented by straight lines with a JMAK exponent n falling in therange from 1.4 to 2.0.The development of crystallographic textures after cold rolling reductions andsubsequent recrystallisation has been investigated. The texture development is shownto 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 thedominance of PSN in HSLA steel, the recrystallisation textures were similar to thatof the rolling textures with weaker intensity of both fibres. To help clarify thereasons for that, in-situ EBSD experiments of recrystallising HSLA steel deformed to50% and 70% have been carried out. It shows that the formation of the nucleusseems to occur within the deformation zones in regions away from the particlesurface 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 thephenomenon of PSN during the recrystallisation process. The simulation shows thediscontinuous evolution of the subgrains in the deformation zone to formrecrystallisation nuclei around the particle. It shows also the subsequent growth ofthese nuclei to consume the matrix region around the particle. The simulation resultsare shown to match with the experimentally observed features of the recrystallisationphenomena in low carbon steel containing coarse cementite particles.

Keyword(s)

Low Carbon Steels; PSN; Recrystallisation; Textures

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