Influence of Slab Reheating Practice on the Microstructural Evolution and Toughness Behavior of Quenched and Tempered Plates of Medium Carbon Microalloyed Steel

Abstract

In the current study, granular bainite was found to be the major component in the microstructure of air cooled 80 mm thick plates of medium carbon microalloyed steel. The second constituent in this granular bainite was identified as cementite. It was further observed that (1) ferrite lath size and (2) amount of cementite in granular bainite varied with slab reheating time before plate rolling. Smaller ferrite laths and a lesser amount of cementite were found in the plate processed with the longer slab reheating time of 26 h. Contrary to this, very large sized ferrite laths and a larger population of cementite were formed in the plate processed with the shorter slab reheating time of 4 h. Subsequent quenching and tempering of these plates favored the formation of lower bainite and tempered martensite in the plate with 26 h slab reheating time. On the other hand, upper bainite and coarser cementite were formed after the quenching and tempering of the plate with 4 h of slab reheating time. The influence of different microstructures, formed due to varied slab reheating time, on the toughness property of tempered plates was evaluated under different test conditions. In tensile test and fracture toughness testing of thinner specimens, a ductile mode of fracture was observed, irrespective of varied microstructures in the tempered plates. However, in the three-point bend test of full thickness specimens, the mode of fracture was ductile in the tempered plate with 26 h slab reheating time, while the tempered plate from the slab with 4 h reheating time gave rise to a predominantly brittle mode of fracture. These observations showed that the toughness property of these tempered plates was sensitive to the microstructure only under the specific condition, which prevailed during the three-point bend test of full thickness specimens. Under this condition, coarse cementite and upper bainite became prone to cracking resulting in a lower toughness of the tempered plate associated with lower slab reheating time.