Effect of process parameters on the mechanical properties of a 45kg grade C-Mn hot rolled steel sheet

Mill sample observations have revealed that presence of abnormal microstructures, such as Widmanstatten and acicular types of ferrites, is responsible for rejection of a 45 kg grade hot rolled steel, SAPH 440, because of its inferior mechanical properties. The effects of manufacturing process parameters have, therefore, been investigated in this study using dilatometry and thermal simulation technique, with the aim of improving the uniformity and strength/ductility balance of the steel. Increase in the finish-rolling temperature was observed to form coarse grained austenite, which transformed to coarse polygonal ferrite or Widmanstatten ferrite at lower cooling rates and/or higher coiling temperatures, while transformed to bainite at higher cooling rates and/or lower coiling temperatures. The former two microstructures were observed to be detrimental to strength, while the latter one was observed to be detrimental to ductility. A microstructure comprising fine polygonal ferrite and dispersed pearlite was noted to yield an optimum balance of strength and ductility, which could be obtained by employing a lower finish-rolling temperature and an intermediate cooling rate. Mill trials have proved that the homogeneity of the mechanical properties and production yield of SAPH 440 steel were improved through proper control of finish-rolling temperature and subsequent cooling process.