Effect of epitaxial ferrite on yielding and plastic flow in dual phase steel in tension and compression

A low carbon, microalloyed steel was heat treated to obtain dual phase micr ostructures containing constant levels of 18 and 25 vol.-% martensite at tw o levels of microstructural refinement and with varying epitaxial ferrite c ontent. Tensile and compression tests were conducted at a strain sensitivit y of 2 x 10(-5). Elastic limits in tension and compression were indistingui shable and very low, suggesting that mobile dislocations were present in th e ferrite as a consequence of stress relaxation processes. These mobile dis locations accommodated the volume increase accompanying the austenite to ma rtensite transformation during heat treatment. Epitaxial ferrite had little effect on the 0.2% proof stress, but average proof stresses were generally higher in compression than in tension owing to residual stresses in the ma rtensite and ferrite following heat treatment. The residual stresses calcul ated from this asymmetry in the proof stresses were small because of stress relaxation in the ferrite at the temperature at which the martensite forme d. Epitaxial ferrite significantly increased uniform elongation in tension with a small decrease in tensile strength both levels of martensite in the finer microstructure but only at the 18 vol.-% martensite level in the coar ser microstructure. The cause of the increased ductility was the effect of epitaxial ferrite on the work hardening rate between approximately 0.5 and 3% strain; epitaxial ferrite reduced the work hardening rare in this range of strain. MST/4350.