Abstract
The influences of precipitates morphology in Ti-added ultra-low carbon sheet steels with carbon content reduced less than 0.001%, in order to clarify the relationships between the r-value and Mn content, and slab reheating temperature, was closely investigated from the viewpoints of gain size of hot band and grain growth during annealing. The obtained results are as follows;
(1) The mean r-value in 0.05% Mn steel improves with dropping the slab reheating temperature (SRT) and shows the highest value at 1050°C. Contrary, the mean r-value in 0.35% Mn steel remains almost a constant even if the slab reheating temperature changes.
(2) Precipitates after hot rolling in 0.05% Mn steel is TiS at a SRT of 1050°C, while fine Ti4C2S2 at a SRT of 1200°C, which is assumed to precipitate during and/or after hot rolling. Contrary, coarse MnS is much observed in 0.35% Mn steel after slab reheating at 1050 and 1200°C, and consequently after hot rolling.
(3) The change in mean r-value as functions of Mn content and SRT can be explained by the changes in hot band grain size and grain growth in relation to the precipitation behavior such as sulfides during slab reheating and hot rolling process.
