Causes of Pinhole Generation on Solidification of Low Carbon S-Pb Free-cutting Steel Cast Continuously

Abstract

In this study, in order to prevent occurrence of surface cracks caused by pinholes occurring in production of low-carbon S-Pb free-cutting steel using continuous casting, on the causes of pinholes generation in this steel, the generating behavior of bubbles in front of solid/liquid interface and engulfment behavior of bubbles by solidifying shell has been theoretically examined from the viewpoint of transport phenomena, thermodynamics, and interfacial science, and the cause of pinhole generation on solidification has been clarified.

Since the oxygen concentration of molten steel is high for undeoxidization to ensure machinability, the CO partial pressure at solid/liquid interface is significantly high due to solute enrichment on solidification. And additionally it is presumed that bubbles tend to be generated at the interface because the total gas pressure increases due to evaporation of lead. Furthermore, it was estimated that this steels are extremely high in sulfur concentration, and the surface tension at solid/liquid interface greatly decreases due to the enrichment at the interface, which also promotes the generation of bubbles on solidification. The CO partial pressure of the total gas pressure accounted for about 90%, and it was estimated that decreasing the CO partial pressure is most important in suppressing bubble generation by proper control of carbon and oxygen content of molten steel and application of stirring of molten steel.

Furthermore, it has been clarified that detachment of the bubbles from the solid/liquid interface is suppressed by the interfacial tension gradient caused by the concentration gradient of sulfur and/or oxygen.