Improvement in internal quality of boiler plates at Bhilai Steel Plant

Bhilai Steel Plant produces plates of boiler, high tensile and other specia l quality steels via the basic oxygen furnace-vacuum are degassing-continuo us casting route. These plates need to be ultrasonically sound. However, re jection of plates on account of internal defects was high. Metallurgical in vestigations of defective plates showed the presence of a martensitic zone associated with cracks around the centreline. The martensitic transformatio n is favoured by segregation of carbon and other elements. The published li terature confirms that, even at a normal cooling rate, martensite formation is possible when the concentration of carbon at the central region is high . Bhilai Steel Plant slab casters have certain inherent deficiencies such a s a high roll pitch and low machine rigidity, which lead to strand bulging. High strand bulging causes centreline segregation. Segregation during soli dification can be controlled by various techniques including low superheat casting, electromagnetic stirring, mechanical soft reduction, and thermal s oft reduction. As a major revamping of the easter and the introduction of a dditional hardware call for a large capital investment, it was planned to i mprove the cast product quality using a less capital intensive option, ther mal soft reduction. In thermal soft reduction, cooling is intensified dose to the final stage of solidification. This creates a thermal compaction whi ch allows the segregated liquid to move upwards and dissipate. The intense cooling also strengthens the solid shell, leading to a significant reductio n of bulging and thereby reducing macrosegregation. In the present investig ation, the secondary water distribution pattern has been redesigned to inco rporate intense cooling close to the final stage of solidification, using a mathematical model developed by Steel Authority of India Ltd (SAIL). The m odified water distribution pattern is designed to maintain a high rate of c ooling at the outer and inner surfaces of the solidified shell in the stran d. Plant trials using the modified water distribution pattern have shown a significant improvement in plate quality, confirmed by macro- and microanal ysis of the slabs and plates. I&S/1444 (C) 2000 IoM Communications Ltd.