MICROSTRUCTURAL CHARACTERIZATION OF THE SOLIDIFICATION AND EQUILIBRATED MICROSTRUCTURES OF Nb-BEARING MICRO ALLOYED STEELS PRODUCED BY THE COMPACT STRIP PROCESSING

摘要

A systematic study of the microstructural characteristics of the solidification and equilibrated microstructures of two commercial low-carbon microalloyed thin slab cast steels produced by Nucor (0.059%C, 0.035% Nb) and HYLSA (0.069%C, 0.063%Nb) was conducted. Samples sectioned from the continuously thin slab cast steels were water quenched (WQ) to room temperature from about 1100℃ either prior to or after equilibrating in the tunnel furnace. Of particular interest in this study was the evaluation of the secondary dendrite arm spacing (SDAS); the effect of Nb content on SDAS; the type, size and composition of the precipitates; and the austenite grain size. The results indicate that the SDAS decreases with increasing Nb content. The average SDAS value from the edge to the center of the slab varied approximately from 40 to 250 μm in size. Two major types of precipitates were observed, TiN and complex star-like (Ti,Nb)(C, N) precipitates. These precipitates were observed at interdendritic sites and in the matrix. The average austenite grain size prior to equilibrating in the tunnel furnace was about 750 μm for the low-Nb steel and 635 μm for the high-Nb steel. The average austenite grain size after the tunnel furnace was approximately 680 and 623 μm, respectively. The average austenite grain size did not seem to be strongly influenced by the equilibration in the tunnel furnace nor by the Ti and/or Nb content of the steels.