Investigation and Minimization of Slag Spot Surface Defects in Continuous Casting of High Carbon Steel Billets through Statistical Evaluation

Slag spot surface defects often appear during continuous casting of high carbon steel billets due to the solidification characteristics of molten steel in the mold. To target the problem of surface slag spot defects that occur frequently during the continuous casting of high-carbon steel strands, we analyzed the influence of molten steel superheat, accumulated service time and the water inlet temperature of the mold, the size of the submerged entry nozzle and the physical and chemical properties of the mold powder on the slag spot defects. The production practice shows that by adjusting the superheat of molten steel to 30–35 °C, the water inlet temperature of the mold is stable at 33–35 °C. To adjust the internal and external diameter of the immersion nozzle to 30–70 mm, the viscosity and melting temperature of the mold powder were adjusted from 0.45–0.55 Pa·s, 1100–1140 °C to 0.15–0.25 Pa·s, 1020–1060 °C. The final billet surface quality was improved significantly, the billet surface was smooth, the oscillation marks were relatively smooth and regular and the slag trench ratio was reduced from the original maximum of 40–50% to less than 1%.

Characterization of inclusions in axle steel by ingot casting

The composition, size distribution and number density of inclusions in the rolled billet of bottom-teemed axle steel have been investigated through the inclusion automatic analysis system (INCAFeature, Oxford Instruments). Additionally, the thermodynamics calculation of sulfide precipitation during the solidification and rolling processes has been carried out. Seven types of inclusions are found in rolled billets, which are almost originated from the refined molten steel. However, re-oxidation, sulfide precipitation and mold powder entrainment modify the composition of inclusions during the pouring, solidification and rolling processes. Precipitation of CaS is initiated during the solidification process of molten steel. The precipitation of MnS can only be found during the hot rolling process. Inclusions in the interior of the rolled billet are more and larger than those in the subsurface of rolled billet. The inclusion index increases from the rear to the head of the rolled billet (from the bottom to the top of the ingot). Furthermore, the inclusion index increases as the superheat increases. On the basis of the results, it is recommended that molten steel should be poured at a low superheat under the protection of argon shroud. Furthermore, attention may be paid to the way of adding the mold powder.