連鋳鋳片のα相析出制御による高温延性改善

抄録

In order to clarify the correlation between susceptibility to transverse cracking and microstructure of slab surface, an apparatus and a method of new hot tensile test were designed. Tensile specimens in a cold crucible type heater were in-situ remelted preceding the deformation. Two thermal history for simulating solidification process, mild cooling and SSC (Surface Structure Control) cooling, were examined. Mild cooling means gradual cooling after solidification corresponding to conventional secondary cooling. SSC cooling is a trial cooling pattern aimed at microstructure control which provides rapid cooling until the γ-α duplex phase region and reheating up to γ region.
The results obtained are summarized as follows.
(1) It was possible to evaluate hot ductility of slab surface with microstructures by means of this hot tensile tests simulating the solidification process.
(2) By SSC cooling, the ductility is significantly improved and the fracture mode changes into transgranular ductile, because film-like ferrite is restrained.
(3) Susceptibility to transverse cracking could be reduced with this microstructure.
(4) Decrease of transverse cracking susceptibility and microstructure control result from fine precipitates dispersion, such as (Ti, Nb)(C, N), caused by SSC cooling.
(5) By utilizing carbonitride precipitation, film-like ferrite along γ grain boundary could be restrained and idiomorphic ferrite could be formed throughout the matrix.
(6) Remelting of specimen before deformation is indispensable to evaluate cracking susceptibility on the hot tensile test.