Improvement in internal quality of boiler plates at Bhilai Steel Plant

Abstract

Bhilai Steel Plant produces plates of boiler, high tensile and other special quality steels via the basic oxygen furnace–vacuum arc degassing–continuous casting route. These plates need to be ultrasonically sound. However, rejection of plates on account of internal defects was high. Metallurgical investigations of defective plates showed the presence of a martensitic zone associated with cracks around the centreline. The martensitic transformation is favoured by segregation of carbon and other elements. The published literature 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 as a high roll pitch and low machine rigidity, which lead to strand bulging. High strand bulging causes centreline segregation. Segregation during solidification can be controlled by various techniques including low superheat casting, electromagnetic stirring, mechanical soft reduction, and thermal soft reduction. As a major revamping of the caster and the introduction of additional hardware call for a large capital investment, it was planned to improve the cast product quality using a less capital intensive option, thermal soft reduction. In thermal soft reduction, cooling is intensified close to the final stage of solidification. This creates a thermal compaction which allows the segregated liquid to move upwards and dissipate. The intense cooling also strengthens the solid shell, leading to a significant reduction of bulging and thereby reducing macrosegregation. In the present investigation, the secondary water distribution pattern has been redesigned to incorporate intense cooling close to the final stage of solidification, using a mathematical model developed by Steel Authority of India Ltd (SAIL). The modified water distribution pattern is designed to maintain a high rate of cooling at the outer and inner surfaces of the solidified shell in the strand. Plant trials using the modified water distribution pattern have shown a significant improvement in plate quality, confirmed by macro-and microanalysis of the slabs and plates.