Grain refinement of C-Mn steel to 1 μm by rapid cooling and short interval multi-pass hot rolling in stable austenite region

Abstract

A noble grain refinement method to attain ferrite grain size of 1 μm in plain C-Mn steel by multi-pass hot rolling in a stable austenite temperature region has been explored. 0.1%C-1%Mn and 0.16%C-0.7%Mn steels were finish-rolled at about Ae3 and then rapidly cooled. Special emphases have been placed on the interval, Δt, between the finish of rolling and the start of water spray cooling of more than 1 000°C-s-1 in cooling rate and the interval between the last two rolling passes. By reducing Δt from 0.5 to 0.05 s alone, the grain sizes near the sheet surface and in the central region were markedly reduced to 1.3 and 2.2μm respectively. By reducing Δt and the rolling pass interval together, a further reduction in grain size was then achieved. Consequently, the steel sheet in which the ultrafine grain structure of about 1μm in grain size penetrated from the sheet surface to the depth of a quarter of the sheet thickness and the grain size in the central region was well below 2 μm was obtained. The transformation to the ultrafine ferrite is considered to be static and from deformed γ. The detailed grain refinement mechanisms are discussed based on the microstructure observation of quenched C-Mn steels and a fcc Ni-30%Fe alloy as well as the calculation of prior γ texture by the misorientation distribution function method. © 2008 ISIJ.