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.
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