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

The effect of cooling pattern and coiling temperature on the mechanical pro perties of high strength dual phase and ferrite-bainite steels was investig ated 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 t he bainite transformation at moderate cooling rates (40 K s(-1)) in order t o obtain a ferrite-martensite microstructure at low coiling temperatures. C hromium and molybdenum are added to obtain high strengths and low yield str ess, 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 temperatur e (CT) was varied from 500 degreesC to room temperature and two different c ooling patterns were used: interrupted and continuous cooling. Differences in mechanical properties and microstructure were observed, but all the stee ls typically show a clear change in mechanical properties in the 200-350 de greesC CT range. Below this range, the yield stress is low, and the tensile strength increases slightly with decreasing CT. Hot rolling finishing temp erature is shown to be important for the mechanical properties, through its strong influence on microstructure ferrite content. Finishing temperatures > 800 degreesC result in a strong decrease of ferrite content. (C) 2001 lo M Communications Ltd.