Effects of Cooling Rates after Hot Rolling on the Planar Anisotropy of Extremely Low Carbon Niobium Cold Rolled Steel Sheets

Abstract

The effects of the cooling rates after hot rolling on the planar anisotropy of cold-rolled Nb steel sheets with extremely low C (C-0.004%, Nb-0.12%) were examined. The specimens were hot-rolled to 3.2mm and cooled with various cooling rates; quenching in water, rapid cooling with water spray, air cooling and furnace cooling.
The sheets were cold-rolled to 0.8mm and annealed at 780°C for 5 hr. The direction of earing at conical cup test varied from diagonal to rolling-transverse according to the decrease of cooling rate after hot-rolling.
To clarify this phenomenon, specimens were exposed in air for several kinds of interval after hot-rolling and quenched in water.
Specimens quenched from γ state showed the earing of diogonal direction and those quenched from a state showed the earing of rolling-transverse direction.
At conical cup test after cold-rolling and annealing, the textures of the sheets of diagonal earing were (554)[225] and those of rolling-transverse directional earing were (111)[110] and (111)[112].
The difference of earing direction can be attributed to the difference of texture formation which is affected by the particle size of precipitated Nb carbonitride in hot-rolled sheets.