Formation of Mono-Layer Honeycomb Structure in High-Purity Iron by Single Pass Hot-rolling

Microstructure change in high-purity irons was investigated at various cooling rates after single pass hot-rolling. Large columnar grains, which are named mono-layer honeycomb grains, developed through the thickness when high-purity iron (C:1.5 mass ppm, N:0.8 mass ppm, S:1.5 mass ppm, Si + Mn + P < 1.0 mass ppm) was hot-rolled at 1273 K and cooled in a furnace with the average cooling rate of about 8.5 × 10 -3 K/s. Mono-layer honeycomb grains developed in neither high-purity iron water-quenched after hot-rolling, nor middle-purity (C: 1.5 mass ppm, N: 9 mass ppm, S: 10 mass ppm, Si+Mn+P: 42 mass ppm) and low-purity (C: 20 mass ppm, N: 8 mass ppm, S: 10 mass ppm. Si +Mn+ P: 270 mass ppm) irons. The crystal orientation of mono-layer honeycomb grains was determined by electron back scattering pattern method. The crystal orientation of mono-layer honeycomb grains does not have any specific α-texture. Grain growth behavior was investigated by using the cold-rolled and annealed irons. The grains in the high-purity iron grew faster and larger than those in middle-purity and low-purity irons. It is considered that the hot-rolled γ-grains in the high-purity iron recrystallize and coarsen in cooling, and then the γ-grains transform into α-grains. The α-grains grow larger due to the effect of purification. Consequently, mono-layer honeycomb grains are developed in high-purity iron.