Microstructure Evolution and Mechanical Property of Cu-Bearing High-Strength Steel Produced by Simulated Strip Casting

Low- and high-Cu high-strength steels are designed and cast through strip and ingot casting. The results show that the Cu suppresses the formation of polygonal ferrite (PF) and intragranular acicular ferrite (IAF), but promotes the formation of granular bainite (GB) and bainite ferrite (BF). The microstructure is refined by the high cooling rate as indicated by the smaller width and length of acicular or lath ferrites for the strip casts. For the high-Cu strip cast, most Cu is retained in solid solution above equilibrium concentration due to the rapid secondary cooling rate. And the rest of Cu is found in the form of Cu-bearing precipitates, i.e., duplex sulfide-oxide particle, shell-like sulfide, and Cu-rich precipitate. However, only duplex sulfide-oxide particle can be found in the low-Cu strip cast. Most of Mn is captured between the dendrites because of the rapid solidification, leading to an evident Mn microsegregation in the interdendritic region of the strip casts. It is found that the number density of nano-scale precipitates in both two strip casts is much lower than in ingot casts, which can be attributed to the incomplete precipitation caused by the rapid secondary cooling rate during strip casting. For either the low- or high-Cu high-strength steels, a higher tensile strength of the strip casts than the ingot casts results from solution strengthening, dislocation strengthening, and fine-grain strengthening. It is noted that the increasing of Cu content from 0.01 to 0.52 wt pct reduces the strength and total elongation of the ingot or strip casts, because the predominant microstructure of IAF turns into BF.