Technological Intervention for Production of High Carbon Billets

Abstract

Substantial market exists for high carbon grades having 0.50–0.65 wt% C for agricultural, automobile, and wire drawing application. However, attributes of cast product in terms of internal and surface/subsurface quality have to be achieved for the required end applications. For production of high carbon grades, the process route followed at Durgapur Steel Plant is through BOF-LTS/LF-Billet Caster. In order to control total oxygen (O) and inclusion level, process modifications were implemented to deoxidize the slag and metal during tapping and at LF/LTS. For proper castability, control of [Mn] and [Si] to achieve proper Mn/Si ratio and O level in steel were standardized. Caster parameters like casting speed, Mn/S ratio, electromagnetic stirring (EMS), lubricating oil, mould, and secondary cooling were optimized to consistently achieve acceptable billet quality. Quality of cast billets with respect to surface defects (laps/bleeds)/subsurface defects (off-corner crack and pinholes/blowholes) were evaluated. Evaluation of internal quality like reheating crack, equiaxed zone (EAZ), and porosity were also carried out. Nitrogen (N) mapping at different stages of steelmaking and refining has revealed that a sharp increase in nitrogen level occurs at ladle stage for high carbon grades as compared to normal medium carbon grades. The surface quality is occasionally marred by the presence of laps and bleeds. It has also been established that EMS current lower than 250 A is inadequate, while higher current on the other hand creates laps and bleeds on billet surface. In order to achieve the requisite product attributes, various recommendations have been implemented. Laps and bleeds are minimized when S < 0.030%, Mn > 0.8%, and casting speed <2.2 m/min. Sound billet quality in terms of central looseness and porosity is possible to achieve when the EMS current is maintained in the range of 270–300 A. Thus, with use of appropriate measures in chemistry control, secondary refining, and continuous casting, high carbon billet of acceptable quality can be produced.