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Abstract
A commercial type dual-phase steel has been heat treated to develop a conventional dual-phase structure and, by a double-quench heat treatment, a dual-phase structure with a small martensite island size. These specially heat treated materials together with the normalized material have been plastically deformed by rolling to a reduction of 98% (εt = 4·0). The tensile properties have been determined after deformation and correlated with the microstructure. It has been found that within the strain range εt = 0·5–1·5 the work hardening modulus is similar to that of pure iron. Over a narrow strain range little work hardening occurs but within the range εt = 2·5–4·0 the work hardening modulus is greater than that of ferrite. The increase in modulus seems to be associated with the plastic deformation of the martensite islands which, at the highest strains, give a fibre reinforcing effect. The results are discussed in relation to the work hardening mechanisms involved. It is concluded that changes in the ferrite grain size, established during the development of deformation bands at lower strains and subsequently deformed at higher strains, greatly influence the flow stress through a Hall-Petch relationship.
MST/241