Degree Name

Doctor of Philosophy


Department of Materials Engineering


Two aspects of mechanical property improvement in a modern low carbon copper bearing high strength low alloy (HSLA) steel have been investigated. These aspects are precipitation hardening by fine copper rich particles and, secondly, the production of fine packets of bainitic and martensitic structures with retained austenite films.

The investigation involved a detailed study of the isothermal and continuous cooling transformation characteristics and microstructural evolution in commercially hot rolled samples of CR HSLA80 steel, which is a low carbon low alloy variant of the ASTM A710 type structural steel. The TIT- and CCTdiagrams were constructed by a combination of dilatometry and metallographic methods.

It has been found that the austenite stabilising elements of copper, nickel and manganese significantly suppress the decomposition of austenite to lower transformation temperatures resulting in a prominent transformation region for bainitic structures, at temperatures intermediate between those of diffusional product (polymorphic ferrite) and the displacive transformation to martensite. The intermediate transformation range involves two typical stages: (i) the formation of a bainitic matrix at the early stage of transformation and (ii) decomposition of remaining carbon enriched austenite regions at lower temperatures.

In the intermediate region, the microstructures were largely characterised by a bainitic matrix containing a high dislocation density, together with a minor dispersed "island" phase. Either on continuous cooling or a short isothermal holding time (5 seconds) at intermediate temperatures (580-430 °C), the island phase was identified as untempered twinned and lath martensite, autotempered twinned and lath martensite, and martensite/austenite constituent, depending on the level of carbon partitioning in the remaining austenite before quenching in water. For a longer isothermal holding time, the carbon enriched austenite regions decomposed to carbide and ferrite by coupled growth. Polygonal and quasi-polygonal ferrite were observed to grow across and eliminate the prior austenite grain boundaries at relatively high transformation temperatures. These structures contained low dislocation densities and e-copper precipitates formed by an interphase transformation mechanism.

At a cooling rate ranging from 0.35 to 20 °C/s, the structure was characterised by a mixture of quasi-polygonal ferrite, Widmanstatten side-plate ferrite, and bainitic structures associated with minor dispersed islands of martensite and/or retained austenite which were dark etching on preparation for optical microscopy. This microstructure develops by the following processes. The Widmanstatten side-plate ferrite nucleates from the ferrite grain boundary allotriomorphs at the early stage of transformation, together with the bainitic ferrite plates which nucleate directly at the prior austenite grain boundaries. On further cooling, the neighbouring plates of Widmanstatten ferrite and bainitic ferrite each tended to coalesce and the volume of untransformed austenite decreased and the shapes of the yγ particles evolved into residual islands between the ferrite plates. Provided the cooling rate was greater than 20 °C/s, the bainitic ferrite plates nucleated directly at the prior austenite grain boundaries, and the plate morphology was revealed by regions of elongated retained austenite or its decomposition products.

At the fastest cooling rate obtained by dilatometry in this work (~375 °C/s), the structure was largely characterised by a mixture of bainitic ferrite and martensitic packets surrounded by retained austenite films. Dilatometric and metallographic examination of the martensite and bainitic ferrite formed on rapid cooling failed to find a clear microstructural distinction between the two products. The packets of bainitic ferrite plates were generally nucleated directly from the prior austenite grain boundaries, whereas, the martensite was characterised by thinner ferritic units with a higher dislocation density. There also appeared to be a larger number of variants of lath packets and apparent intragranular nucleation in the case of martensitic ferrite.

The general relationship between transformation on continuous cooling and transformation at constant temperature was also studied in the low carbon ferritic steel. It was found that the progress of isothermal ferritic transformation kinetics can be well expressed by the Avrami type equation (X=1-exp{-K(T)tn}). Considering the continuous cooling to be a series of isothermal steps, the temperature dependence of K(T) is determined from the isothermal kinetics data, assuming that "n" is independent of temperature. Using this expression, a kinetic equation for austenite decomposition during cooling was derived by extension of the additivity rule to the whole range of fractional completion. It has been shown that there is a good agreement between the measured and calculated CCT-curves.

The experimental results indicated that the strength of CR HSLA80 can be improved substantially by copper precipitation, and that the potential of copper based precipitation hardening is significantly affected by the pre-aging treatment. The copper age hardening response was investigated at temperatures of 450, 500, and 550 °C for three pre-treated structures (as rolled ferrite, bainitic ferrite, and martensite).

The aging results reveal that the age hardening responses of both the martensitic and.bainitic ferrite structures are higher than that of the as rolled steel, and this observation is rationalised in terms of higher solute Cu content, and a higher density and greater uniformity of dislocations which provide a multitude of nucleating sites for copper precipitation. Moreover, it has been found that the peak hardness in the martensitic and bainitic structures was attained when copious amounts of fine ε-copper precipitates were observed on dislocations. Compared to the martensitic and bainitic cases, the presence of pre-existing interphase ε-copper precipitates, as well as the formation of additional copper-rich clusters and precipitates from supersaturated ferrite contribute to the aging response in the hot rolled condition.

Examination of the commercial hot rolled samples of CR HSLA80 steel indicated that relatively coarse interphase copper precipitates formed in the hot rolled steel plate during austenite to ferrite transformation on cooling from the rolling finish temperature and thus the full strengthening potential of the alloy was not achieved by the subsequent aging treatment. Higher strength levels can be obtained by cooling rapidly to temperatures lower than 580 °C to suppress interphase precipitation on cooling, producing Cu supersaturated bainitic or martensitic structures, and then aging to produce fine multivariant copper precipitation in the matrix.