含Nb低炭素鋼における熱延時オーステナイトの再結晶抑制効果

抄録

An investigation was made of the effect of Nb on the refinement of ferrite grain size in hot-rolled low-carbon steels.
Experiments of hot-rolling by 2-passes were performed using 11 vacuum-melted steels with various C, N, Nb, and V contents. The main results are as follows:
(1) In Si-Mn steels not containing such special elements as Nb, the coarse grains of austenite at soaking temperature are recrystallized into fine grains during hot-rolling. The relation between the austenite grain size just above Ar₃ and the transformed ferrite grain size is similar whether the process contains hot-rolling or not.
(2) In the steels containing comparatively large amounts of Nb or, when hot-rolled at relatively low temperatures, in the steels containing small amounts of Nb, the austenite can not recrystallize during cooling from finish-rolling temperature to Ar₃ and transforms to ferrite of finer grains. Ferrite grains nucleate around the deformation bands in unrecrystallized austenite grains as well as at the austenite grain boundaries.
Mixed grain and/or upper-bainitic structures sometimes appear after transformation.
(3) The retardation of recrystallization of austenite during hot-rolling process in Nb-containing steels is due to NbC (N) which has been in solution at soaking temperature and finely precipitates during hotrolling. Hot-rolling accelerates such precipitation. Therefore, when finish-rolled at a higher temperature, the austenite of steels containing small amounts of C, N, and Nb recrystallizes completely.
(4) When the austenite recrystallizes completely during cooling to a temperature just above Ar₃, the presence of precipitates, as NbC (N) or VN, can contribute to the refinement of ferrite grain size, probably by retarding the growth of ferrite grains.