A Homogenization Technology for Heavy Ingots: Hot-Top Pulsed Magneto-Oscillation

We scaled up a previously developed method, known as Hot-top Pulsed Magneto-Oscillation (HPMO), to minimize crack, shrinkage-cavity, and macrosegregation in large ingots. Simulations on electromagnetic field, flow field, and temperature field revealed that an HPMO-induced electromagnetic field forces circulation of liquid steel near the riser, which causes grain nuclei and free grains to fall off the riser walls, drift away, and settle onto the middle of the mold. This phenomenon, known as “grain showering”, refines solidified structure and reduces segregation. Joule heating generated by the HPMO process leads to mass feeding of the riser, which eliminates the development of pipes or central porosity and cracks. Based on simulation results, we designed a prototype HPMO apparatus and tested it on the production of ingots weighing 18 tonnes. Experimental results indicated that the use of HPMO grain showering did indeed yield expected solidified structure in ingots with a 56–83 pct reduction in equiaxed grain size, a 41 pct reduction in the number of inclusions, and a 50–75 pct reduction in normal carbon segregation comparing with that in controlled ingots. Furthermore, by using HPMO, shrinkage-induced pipes and center cracks that often occurred in the control ingots were eliminated, resulting in a fourfold increase in ductility in critical regions of the ingot. This work demonstrated the value of computation-aided simulation for improving manufacturing methods for the casting of large ingots.