A study has been made of the effects of the cooling rate during solidification and the charging temperature prior to hot-rolling on the mechanical properties of multi-phase high strength hot-rolled sheet steels.The strength of B-bearing multi-phase sheet is independent on the cooling rate during solidification and decreases with dropping the charging temperature prior to hot-rolling. That may be due to the decrease in solute boron content segregated to austenite grain boundary to improve hardenability, because of the precipitation of BN during cooling the ingots below 1000°C.
The strength of Nb-bearing multi-phase sheet steel decreases with dropping hot-charging temperature.The decrement occurrs at lower temperature in the hot-rolled sheet produced from the ingot solidified in cast-iron mold to simulate the solidification condition of thin-slab casting than in sand mold sample to simulate conventional continuous casting, an effect which is explained in terms of effective suppression of NbC precipitation during cooling in the cast-iron mold ingot.
