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Microalloying additions are made to low carbon steels for a number of purposes. In the conventional controlled rolling of reheated slabs, an important function of niobium is to control recrystallisation in the austenite phase field in order to refine the ferrite grain size of the controlled rolled product. A second benefit is that niobium makes a contribution to the strength of the product through precipitation in the ferrite. Vanadium carbides and nitrides are too soluble in austenite to have a significant effect on grain refinement, when added to a Nb-bearing steel, but may augment precipitation strengthening, particularly at raised nitrogen levels. Titanium is generally added to restrain grain growth in the austenite on reheating and in the heat affected zone of welds, but is also capable of restraining recrystallisation in the austenite during controlled rolling, and of contributing to tensile strength. In conventional processing of continuously cast thick slab, the steel is reheated to 1 200–1 250°C for up to 8 hr before rolling. This conditions the austenite by partial homogenisation and solution of the microalloying elements. The austenite grain size after soaking and at the start of the rolling process is then usually smaller than it was in the original casting, being determined by the dispersion of any undissolved microalloy nitrocarbides and undissolved inclusions. In recent years, however, the practice of hot direct rolling has been developed, in which the as-cast austenite is rolled directly, without the intermediate stages of cooling to a low temperature, reheating and soaking. The austenite then has the coarse-grained, dendritic and segregated microstructure of the original casting prior to the commencement of hot rolling. Although hot direct rolling has been practised using conventional thick slab, its most practical application is in the production of hot rolled strip from thinner slab in minimills. With this newer technology, slab in thicknesses down to 50 mm is continuously cast and directly hot rolled within, typically, 15–20 min after casting; the only “heat treatment” between casting and rolling is temperature equalisation at approximately 1 100–1 150°C during that time interval. Both niobium and titanium segregate to the liquid phase during freezing, to form eutectic carbonitrides or nitrides on solidification. In the absence of a prolonged soaking period at high temperature, it might be expected that the presence of such precipitates in the austenite at the commencement of direct hot rolling might reduce the effectiveness of the microalloying additions, compared to their effect in conventional processing. The purpose of this research, therefore, was to investigate the tensile properties of hot direct rolled low carbon steels microalloyed with niobium, the composition being varied to allow eutectic NbCN to form over part of the range. The effect of including V and Ti additions over part of the composition range was also investigated.

Tensile Properties of Simulated Thin Slab Cast and Direct Rolled Low-carbon Steel Microalloyed with Nb. V and Ti.