INFLUENCE OF INTERCRITICAL ANNEALING ON THE MECHANICAL PROPERTIES OF PLAIN-CARBON DUAL-PHASE STEELS

Abstract

Dual-phase steels consisting of martensite phase embedded in the ferrite matrix are becoming important engineering mate-rials due to a unique combination of properties viz., (i) continuous yielding behaviour or the absence of yield point phenomenon, (ii) low 0.2 pct. off-set proof strength, (iii) high work hardening rate, (iv) high tensile strength & (iv) high ductility measured as uniform and total elongations. Consequently, although the dual-phase steels have strengths comparable to those of high strengths low alloy (HSLA) steels/ yet their ductility and formability are comparable to those of the mild steels. These high strength steels are, therefore, increasingly used as thin-sheets in the automobile industry for fabrication of light vehicle bodies to achieve appreciable fuel-economy. A critical review of the available literature reveals that most of the studies conducted so far on these steels have either been restricted to low alloy high strength (HSLA) steels or carried out at fixed intercritical annealing times at which the volume fraction of austenite reaches almost the equilibrium value. Little attempt has been made to study plain-carbon dual-phase steels, specially to elucidate the role of their carbon content in a wider range of composition. Further, the formation of austenite takes place through a series of steps and the composition and volume fraction of the .austenite and the matrix keeps on changing before attaining equilibrium. Therefore, a study of the effect of time of intercritical annealing on mechanical properties of dual-phase steels is imperative to get an optimum combination of strength, ductility and formability. In other words, a need for correlation of mechanism of formation of dual-phase microstructure with mechanical properties of such dual - phase steels exists. It is also necessary to reveal the role of variation of carbon content of these steels from very low to high values to analyse its effect on the different stages of formation of austenite and hence martensite on quenching, and therefore, on the overall mechanical properties of plain-carbon dual-phase steels'. For theoretical prediction of strength of dual-phase steels and the impact of various microstructural parameters on it, these have been treated as composites and the law of mixtures has been applied by earlier workers. But no attempt has been made to incorporate the effect of shape of second phase particles embedded in the matrix in such composite materials, and also the effect of strain-hardening. The present investigation has, therefore, been undertaken to, study plain-carbon dual-phase steels in order to gain an insight into the above mentioned areas. The entire work has been presented in Seven Chapters in the thesis. Chapter-1, general introduction of. dual- phase steels is presented with respect to their historical development and paramount importance. Chapter-2 deals with a critical review of the available literature on dual-phase steels. After very briefly reviewing the production techniques of dual-phase steels, the different stages of austenite formation along with its kinetics when conducted non-isothermally have been focussed. The mechanical properties in respect of work-hardening, yielding behaviour, strength, ductility and formability have also been critically examined in the light of current understanding. The review of literature has culminated in identification of specific gaps where this investigation is directed. Chapter-3 deals with theoretical analysis of the strength of dual-phase steels. Contribution of different strengthening mechanisms for example, strain-hardening and superior load bearing capacity of martensite islands have been investigated within the framework of shear-lag model of spherical and cylindrical with hemi-spherical ends of martensite islands. An effort has been made for the first time to take into account the entire range of shapes of second phase martensite particle in terms of the ratio of radius to length, (r/(), varying from 0.4 to 0.6 within the framework of single-particle model