A 3D model applying temperature- and carbon concentration- dependent material properties was developed to describe the scrap melting behavior and carbon diffusion under natural convection. Simulated results agreed reasonably well with experimental ones. Scrap melt-ing was subdivided into four stages: formation of a solidified layer, rapid melting of the solidified layer, carburization, and carburization + nor-mal melting. The carburization stage could not be ignored at low temperature because the carburization time for the sample investigated was 214 s at 1573 K compared to 12 s at 1723 K. The thickness of the boundary layer with significant concentration difference at 1573 K increased from 130 μm at 5 s to 140 μm at 60 s. The maximum velocity caused by natural convection decreased from 0.029 m·s?1 at 5 s to 0.009 m·s?1 at 634 s because the differences in temperature and density between the molten metal and scrap decreased with time.