ON THE APPEARANCE OF A PECULIAR SURFACE LAYER WITH COARSE GRAINS BY THE ANNEALING OF 3S SHEET

Abstract

Various properties of 3S sheet when it was heated in air furnace or salt bath and a structure of its alloy slab were examined. The results obtained were as follows:(1) When 3S sheets, having each thickness 3.2mm (cold reduction degree 46%), 1.6mm (72%), 0.8mm (84%), were annealed in the air furnace, we found that a peculiar surface layer consisted of the very coarse grains had appeared on them. The grains of such surface layers were coasser as the annealing temperature lower and smaller as the heating velocity faster. Also the grain size was maximum near 70% cold reducion degree. In the case of salt bath heating, the difference of grain sizes between surface and inner part could not be seen and the grains were smaller than any ones of the air furnace heating.(2) The thickness of the surface layer was very thin such as only 2% of the sheet thickness and each about 0.06mm (3.2mm sheet), 0.03mm (1.6mm), or 0.01mm (0.8mm). If this layer appeared once, it did not disappear although the sheet was rolled to 0.1mm. And even specimens with such thin peculiar surface layers exhibited orange peal by deep drawing.(3) The surface layer had the higher recrystallization temperature, the lower hardness than inner part and also the distribution of hardness or grain size changed suddenly at certain depth same to the layer thickness, it was hardly etched, probably due to the homogeneous distribution of working stress.(4) As to the microstructure of sheet, Mn compound particles in the surface layer were smaller and distributed homogeneously, but larger and heterogeneous in the inner part. Then, the dispersoid only could be seen in the inner part.(5) The structure of slab surface chilled at the face of metallic mould exhibited the very coarse grains involving the small particles of Mn compound and no dispersoid, but the inner structure appeared with the small grains involving the large particles of Mn compound and a great number of dispersoid. The slab structure like this was proportional to the sheet structure.(6) The sheet worked by cold rolling after preheating at 550°×15hrs exhibited the very small recrystallized grains, perhaps owing to the appearance of dispersoid in surface or inner part.(7) The growth of the peculiar surface layer and the grosscrystallization of recrystallized grains in it were thinked separately through these results.a) As for the growth of the surface layer, the surface is deformed by hot rolling more severely than the inner pard and softened with the heat formation by this severe deformation. This also the crystal orientations of grains are ordered gradually. By cold rolling the surface is deformed more and more with a partially heterogeneous rolling action and the crystal orientation is ordered strongly. Through this process the difference of various properties between surface and inner part will appear as described above.b) About the grosscrystallization of surface grains, we think that a incubating coarsen property of Mn in the alloy will be a important factor for grain growth and this characteristies of Mn will be promoted by the following facters such as: the surface structure of casting, the strongly preferred orientation of grains in the surface layer with particularly severe deformation, and the working process the dispersoid do not appear through rolling period.(8) In order to the grains, preheating prior to the finishing rolling or salt bath heating was effective.