Suguhiro Fukushima

Ultrafine Grained Steels due to Super Short Interval Multi-Pass Rolling in Stable Austenite Region

Ultrafine-grained steel sheets with the chemical composition of 0.15%C-0.74%Mn- 0.01%Si have been prepared using a laboratory rolling mill by Super Short Interval Multi-pass Rolling (SSMR) process, in which the inter-pass time is extremely shortened to enhance the cumulative strain. The SSMR process with a finish rolling around Ae3 leads to ultrafine equiaxed ferrite structure with 1μm in average grain size. In order to clarify the grain refinement mechanism in the SSMR process, the deformation substructure in deformed austenite was simulated using 70%Ni-30%Fe, which was a fcc alloy with equivalent stacking fault energy to C-Mn steels. TEM observations have shown that the dislocation substructure in the Ni-Fe alloy hot-rolled by SSMR process mainly consists of dislocation cells, of which size are refined to less than 1μm with shortening inter-pass time. It is concluded that the SSMR process can accumulate the deformation strain in austenite enough to densely nucleate ferrite inside austenite grains.

Effect of Ultra-Fast Cooling after Rolling in Stable Austenite Region on Grain Refinement of C-Mn Steel

Ferrite grain refinement by hot rolling mostly above Ae3 being followed by an ultra-fast cooling has been investigated. An emphasis has been paid on the interval, Δt, between the finish of rolling and the start of water spray cooling of which cooling rate is more than 1000 °C·s-1. When Δt was nearly equal to zero, ultra-fine ferrite of about 1 or 2 μm in grain diameter was obtained for 1.2 to 1.3 mm thick 0.1%C-1%Mn steel sheets near sheet surfaces or in thickness center regions respectively, although the grain size at Δt of 0.5 s was about 3 μm in both regions. The ferrite grains were almost equiaxed and surrounded by high angle boundaries. This grain refinement is likely to be caused by an increased number of nucleation sites for the transformation from austenite to ferrite due to the ultra-fast cooling. Such a grain refinement mechanism is discussed in terms of prior-austenite structures deduced by the misorientation distribution function analysis.