Tom Waterschoot

Influence of run-out table cooling patterns on transformation and mechanical properties of high strength dual phase and ferrite-bainite steels.

Abstract The effect of cooling pattern and coiling temperature on the mechanical properties of high strength dual phase and ferrite–bainite steels was investigated using four steel compositions (Mn–Cr, Mn–Nb, and two Mn–Cr–Mo based). About 1·5%Mn is required to stabilise the austenite sufficiently to avoid the bainite transformation at moderate cooling rates (40 K s-1 ) in order to obtain a ferrite–martensite microstructure at low coiling temperatures. Chromium and molybdenum are added to obtain high strengths and low yield stress/tensile strength (YS/TS) ratios. Niobium additions make it possible to obtain a ‘dual phase hybrid’ steel, characterised by a finer ferrite phase with precipitation hardening, and a higher YS/TS ratio. Coiling temperature (CT) was varied from 500°C to room temperature and two different cooling patterns were used: interrupted and continuous cooling. Differences in mechanical properties and microstructure were observed, but all the steels typically show a clear change in mechanical properties in the 200–350°C CT range. Below this range, the yield stress is low, and the tensile strength increases slightly with decreasing CT. Hot rolling finishing temperature is shown to be important for the mechanical properties, through its strong influence on microstructure ferrite content. Finishing temperatures >800°C result in a strong decrease of ferrite content.

Neutron diffraction study of the low-temperature ageing of martensite in dual-phase steel

Abstract Neutron diffraction measurements were used to obtain a better understanding of the ageing behaviour in dual-phase ferrite + martensite steels. Therefore, the influence of different ageing treatments on the tempering of low-alloyed 0.4 and 0.7% C FeMnCrMo-martensite and 0.1% C FeMnCrMo dual-phase steels was investigated. On tempering at 170 and 350°C, the sequential precipitation of η- and θ-carbides, as well as the loss of tetragonality was observed in the martensite. The decrease in austenite lattice parameter after tempering is due to stress relaxation in the material. These observations offer an acceptable explanation for the large yield stress increase after ageing at 170°C.

Amplitude-Dependent Internal Friction of Dislocation Interactions in Dual-Phase Steel

Abstract Dislocation interactions in the ferrite matrix of a dual-phase steel have been studied as a function of martensite volume fraction (11–25%) and tensile straining (0–5%) by strain-induced amplitude-dependent (strain amplitude varying from 10−7 to 10−4) internal friction measurements at room temperature, using a high-frequency composite oscillator (40 kHz). Results showed that a higher volume fraction of martensite increases the internal stress in the softer ferrite matrix and restricted the dislocation motions, giving rise to a lower amplitude-dependent internal friction. Application of tensile straining further shortened the effective mobile dislocation length which was revealed by the internal friction measurements and this resulted in a high initial strain hardening rate of the high-martensite samples. Reduction in the martensite volume fraction and the mobile dislocations in the ferrite matrix through process control resulted in an increase in the amplitude-dependent internal friction, indicat...