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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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Dual phase microstructures were produced in a low carbon steel, in which the martensite volume fraction was kept constant at two levels, of 18 and 25%, while the epitaxial ferrite content was varied independently. The microstructures were produced with two dispersions of martensite, a relatively coarse dispersion by intercritical annealing of a ferrite/pearlite starting microstructure and a ® ner dispersion from an initial martensitic microstructure. Bauschinger tests were conducted using prestrains in both tension and compression of 0 . 4, 1, and 2 . 2%. Prestrain direction had no measurable effect on plastic ¯ow behaviour after strain reversal. Mean back stresses increased with increasing strain and martensite content, and were higher for the ® ner martensite dispersion. They decreased signi® cantly with increasing epitaxial ferrite content in the case of the ® ner dispersion, but less signi® cantly in the coarser dispersion. These effects of microstructure are discussed in terms of stress transfer to martensite, work hardening, and tensile properties. It is concluded that about half of the mean back stress developed during early plastic deformation was generated by stress transfer to the martensite, the remainder arising from strain hardening of the matrix. A small permanent softening in the Bauschinger test resulted from a reduction of effective stress in the ferrite matrix when the strain path was reversed.MST/4990

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