Ultrafine grained ferrite/martensite dual phase steel fabricated by large strain warm deformation and subsequent intercritical annealing

An ultrafine grained (UFG) ferrite/cementite microstructure was produced by use of large strain warm deformation in two plain C-Mn steels. In order to overcome the characteristic restricted tensile ductility of this steel, an intercritical annealing was applied to obtain an UFG ferrite/martensite dual phase (DP) steel. Suitable intercritical annealing parameters have been worked out using dilatometry. Microstructure evolution during intercritical annealing has been investigated by means of scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The study revealed that increasing the Mn content from 0.87 to 1.63 mass% was highly beneficial for the formation of martensite. This effect is explained by the enrichment of Mn in cementite which is partly inherited by austenite. The final microstructure consists of martensite islands embedded in an ultrafine grained polygonal ferrite matrix. The average grain size is 1-2 mu m. Small amounts of retained austenite (<1 mu m) are finely dispersed. The grain size is hardly affected by the intercritical annealing, whereas the fraction of high-angle grain boundaries in the ferrite matrix is reduced. Tensile tests revealed that strain hardenability is drastically improved by the introduction of martensite as a second phase. The UFG-DP steel exhibits a good combination of high strength and uniform elongation and considerable strain hardenability.