| The effect of hot rolling condition on the mechanical properties, microstructure, find HIC susceptibility was investigated for microal1oyed steels which were prtpared in the mill lines as well as in the la bo ratory. The results are as follows. (1) The microstructure of hot-rolled steels was hardly affected by the change of the finishing temperature. But by lowering the coiling temperature, the polygonal ferritic and pearlitic structures were changed to the bainitic stucture. (2) With lowering the coiling temperature, the tensile and yield strengths were increased by more than 10 ㎏.f/㎟, while total elongation was decreased by 10%. However, the effect of the coiling temperature on the impact tou ghness was very little. (3) The most important factor affecting the initiation of HIC was the morphology of non-metallic inclusions. Elongated MnS inclusions as well as large and/or clustered oxides rendered the steels very crack sensitive. (4) Cracks propagated along the pearlitic bands or low temperature transformed phases developed in the center-segregated regions of C.C.slabs. HIC susceptibility was decreased to some extent as center-segregation was decreased. (5) The propagation of HIC was reduced when the formation of the low temperature transformed structure and/or the growth of the lamellar pearlite was supressed by proper cooling control. |
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