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Manchester. M1 7HS, UK. A series of Nb-Ti steels containing titanium in the range 0.005 to 0,038wt"/• and nitrogen in the range 0.005 to 0.01 1 wto/• were prepared using a base composition of 0.06wto/• carbon and 0.027 wt"/• niobium.Small ingots were cooled slowly in order to simulate larger castings. Cooling was interrupted at various temperatures in the austenite phase field, either by quenching or by holding isothermally before quenching.Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to investigate the morphology, distribution, composition, and particle size of carbonitrides in processed ingots. Samplesof the quenchedingots were tempered for I hr at 600'C. Temperedhardness

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The attainment of ultra®ne ferrite grain structures in low carbon, low alloy steels is of interest because of the improvement in yield strength and Charpy impact transition temperature predicted by extrapolation of known data to very ®ne grain sizes. This paper presents a summary of research aimed at producing ultra®ne ferrite in a niobium microalloyed, low carbon steel by three processing routes. Transformational grain re®nement (TGR), in which extra®ne austenite is hot rolled and cooled rapidly, has been shown to be capable of producing grain sizes of v1 mm in a surface layer, and 1 . 5 mm in the centre of 3 mm thick plate. Dynamic recrystallisation of ferrite during multipass warm rolling was shown to be neither complete nor uniform within the cross-section of the plate. Nevertheless, a partly recrystallised, partly recovered grain structure with an average grain size of 1 . 5 mm was obtained in the centre of 3 mm thick plate. Cold rolling and recrystallisation of ferrite that had been previously re®ned by TGR to an intermediate grain size was shown to produce an ultra®ne grain microstructure (v1 mm grain size) throughout the section of 1 mm thick strip. The hardness of ultra®ne ferrite was shown to obey a linear relationship with the inverse square root of grain size, but with a lower slope than expected from the Petch relationship for yield strength.MST/4772

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