Year

1987

Degree Name

Doctor of Philosophy

Department

Department of Metallurgy and Materials Engineering

Abstract

Two aspects of mechanical property improvement in vanadium and niobium H.S.L.A. steels have been investigated. They are, dispersion hardening of the ferrite by fine carbonitride precipitates and, secondly, grain refinement of the final ferrite/pearlite structure.

The investigation involved the study of the morphologies of ferrite and carbonitride precipitates in laboratory treated and in commercially hot-rolled samples of vanadium, niobium and vanadium-niobium steels. Ferrite grain morphologies were compared with prior austenite grain structures to elucidate the mechanisms of grain refinement accompanying the polymorphic transformation. Commercial steels were used in this investigation rather than the higher alloy analogues often used in previous investigations.

It was confirmed that fine precipitate dispersions significantly improved the strength of the vanadium and niobium H.S.L.A. steels and that precipitation of these carbonitrides often occurred at the migrating austenite/ferrite interface. Alloy carbonitride precipitation associated with ferrite formation from undeformed (recrystallised) austenite was generally consistent with either the Interphase Precipitation Planar model or Interphase Precipitation Curved model proposed for higher alloy steels by other investigators. These mechanisms, which are respectively associated with coherent and incoherent interphase boundaries, lead to layers of particles with spacings and regularity which vary with transformation conditions. Although a general correspondence was established between precipitate modes in the H.S.L.A. steels and higher alloy steels, the lower volume fraction of precipitate in the H.S.L.A. steels precluded the operation of the "quasi-ledge" mechanism proposed for higher alloy steels.

Where the austenite grain size prior to transformation was large, or the reduction small, complete transformation could not be achieved by the "cascade nucleation" process and coarse intragranular ferrite resulted, giving rise to mixed ferrite grain size structures.

The influence of prior austenite grain size on grain refinement of the ferrite was examined by means of measurements of grain size distributions in austenite and ferrite. Finer ferrite grain structures resulted from deformation and subsequent transformation of smaller austenite grains. This grain refinement did not result principally from a reduction in size of the small ferrite grains nucleated at, and adjacent to, the austenite grain boundaries, but by an increase in the number of these small grains at the expense of coarse intragranular ferrite grains.

Ferrite grain size distributions in both laboratory rolled and commercially rolled steels were heavily skewed to larger grain sizes, but were not consistent with a log-normal distribution. The numerical mean grain size of the ferrite was found to be a poor indicator of ferrite grain structures in these steels and it is proposed that a more appropriate characterisation of ferrite grain structures is provided by mean grain sizes weighted in terms of surface area or volume.

Carbonitride precipitation was found to influence ferrite grain structure development in two ways. Firstly, pinning due to precipitates on the austenite sub-structure, together with solute redistribution and precipitation at the interphase boundary, can exert retarding effects on ferrite growth during the polymorphic transformation, indirectly enhancing grain refinement by allowing activation of additional ferrite nucleation sites. Secondly, in the commercially hot-rolled steels, ferrite grain shape anisotropy was observed which was associated with retardation of ferrite growth perpendicular to the rolling plane by precipitates on the elongated substructure of deformed austenite.

The same modes of interphase precipitation were also observed on transformation of deformed (unrecrystallised) austenite, but in this case the dispersions were characterised by significantly finer particle sizes and sheet spacings, as well as a general loss of regularity of the layered structure.

Examination of the commercially hot-rolled strip indicated that not all of the available vanadium and niobium carbonitride precipitated in the steel during the polymorphic transformation and thus the full strengthening potential was not achieved. Higher strengths were obtained by cooling rapidly to approximately 600°C to suppress interphase precipitation and then allowing fine multivariant precipitation to occur in situ in ferrite during ageing.

Nucleation of ferrite grains was observed to occur preferentially at grain boundaries, deformation bands and deformed twin boundaries. The nucleation frequency at prior austenite grain boundaries was shown to increase rapidly with deformation, even for light reductions, but approached a limiting value at higher reductions. Further transformation proceeded by the "cascade nucleation" process in which renucleation of ferrite occurs at, or just ahead of, the advancing austenite/ferrite interface in the deformed austenite. The extent of "cascade nucleation" was found to increase with increasing reduction.

On the basis of the present work it is proposed that the effects of interfacial drag by precipitates on the austenite sub-structure and the sideways impingement of the growing ferrite grains can combine to temporarily immobilise the advancing ferrite growth front, allowing renucleation of ferrite grains at adjacent austenite subgrain boundaries or, more probably, at the interface between austenite and ferrite grain boundary cusps, arising from grain interface junctions. Higher strains in the austenite favour extension of the cascade nucleation into the grain interior because of the generation of both finer particles which more strongly retard ferrite growth and higher stored strain energy which enhances nucleation of ferrite.

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