Formation of giant columnar grains in high-purity iron by hot-rolling

The microstructure change of three kinds of high-purity irons (HP, MP and LP irons) by hot-rolling at various temperatures was investigated. Columnar grains in high-purity irons develop along the normal direction after hot-rolling in the gamma-phase region, The growth of columnar grains depends on the purity of the iron: the width and height of columnar grains increase with the increase of die purity of the iron. Giant columnar grains of HP-iron grow to the center of the thickness. Columnar grains do not develop by hot-rolling in die alpha-phase region nor by heating in gamma-phase region without hot-rolling. Apparently, the gamma --> alpha transformation after hot-rolling in the gamma-pllase region plays an important role in the growth of giant columnar grains, The crystal orientation of giant columnar grains was determined by an electron back scattering pattern method. The transformed alpha-rexture was calculated from the gamma-texture with (112)(111) by using the Krudjumov-Sachs, relationship. The calculated alpha -texture is in good agreement with the measured crystal orientations except for {100}(001). Furthermore, the formation mechanism of giant columnar grains has been proposed.