Requirements for Developing Cube-on-Edge Texture in 4-mil-Thick Silicon—Iron

Abstract

Cube-on-edge grain oriented silicon—iron strip is widely used in electrical apparatus in thicknesses down to about 10 mils (0.25 mm). Lighter gauge strip, such as is commonly used at frequencies above 60 cps, is made commercially by cold rolling the normal gauge, grain-oriented material to the desired thin gauge and then recrystallizing. The requirements for developing the cube-on-edge texture directly at 4 mils (0.1 mm), as is done at thicknesses of 10 to 14 mils, have been determined using a laboratory-made heat, containing 3.3% silicon, 0.026% sulfur, and 0.06% manganese. The development of cube-on-edge texture by secondary recrystallization depends upon the presence of inclusions to restrain normal grain growth. The effectiveness of inclusions in performing this function depends upon their size, which is determined by the prior thermal history of the material. The driving force for secondary recrystallization is inversely proportional to the radius of curvature of the primary grains. As the thickness of the strip decreases, the fraction of grains with a face on the surface of the strip increases and the average radius of curvature increases. Therefore, to maintain the driving force, the primary grain size must be decreased as the strip thickness is decreased. This is achieved during the prior hot processing of the ingot by increasing the cooling rate from the solution temperature of the inclusions, thereby obtaining a greater number of—and smaller—inclusions. It is shown that with a cooling rate of 130°C/min from the solution temperature, secondary recrystallization occurs in strip 4 mils thick as well as 12 mils. However, with a cooling rate of 50°C/min, secondary recrystallization occurs only in the thicker material. The thinner the strip is the more readily are the inclusions prematurely lost during the texture-developing anneal. It is shown that only normal grain growth is obtained when 4-mil strip is heated in dry hydrogen, whereas complete secondary recrystallization is obtained when a wet hydrogen atmosphere is used. This is attributed to the diffusion barrier presented by a layer of silica on the surface.1