UPPER AND LOWER BOUNDS IN FLOW-STRESS OF A DUAL-PHASE STEEL PREDICTEDBY THE SECANT METHOD AND RELATED MICROSTRUCTURES

Elasto-plastic flow curves of dual phase steels consisting of martensi te and ferrite have been predicted by using the secant method proposed by WENG. The predicted flow stress for a given volume fraction of mar tensite becomes the upper bound when the matrix is considered to be ma rtensite whilst the predicted flow stress becomes the lower bound when the matrix is ferrite. This prediction is compared with experimental results obtained from Mn-Si-C martensite-ferrite steels where the volu me fraction and morphology were altered by changing heat treatment. It is found that the prediction is in good agreement with the experiment al results when the matrix structure can be identified by means of opt ical microscopy. In the case where the matrix structure is percolated and is not distinguishable, the flow stress curves lie between those t wo bounds.