Origin and development of through-the-thickness variations of texture in the processing of grain-oriented silicon steel
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INTRODUCTION
LOCALinhomogeneities of microstructure and texture are shown ~ to be a primary requisite for occurrence and perfection of secondary recrystallization in grain-oriented silicon steel. In the production of high-permeability grain-oriented silicon steel sheets, structural inhomogeneities arise in the hot rolling stage. 2 The inhomogeneities occur in two ways. One is the local irregularity of grain size and precipitate morphology because of the partial phase transformation in the hot rolling and the subsequent annealing stage. The basic composition of commercial high-permeability grain-oriented silicon steel is 3 pct Si and 0.07 pct C. This combination is known to expand the gamma loop, as evidenced by the work of Rickett and Fick 3 who found that a carbon content of 0.07 pct in otherwise pure iron-silicon alloys extended the gamma loop up to 6.5 pct silicon with the maximum amount of austenite at about 1423 K. The other source of inhomogeneity is the texture gradient through the thickness of hot-rolled sheet; the preferred orientation varies from {110} (001) in the subsurface and intermediate layer to {001} (110) in the midplane of sheet. These orientations are related by a 90 deg rotation about the transverse direction. A good combination of the microstructural and the textural inhomogeneities provides the nuclei for secondary recrystallization in the Goss orientation, which manifest themselves in the form of coarse grains in the {110} (001) orientation and located at a depth of 20 to 25 pct of the thickness below the surface of the hot-rolled sheet. MUNETSUGU MATSUO is Chief Researcher, Nippon Steel Corporation, R & D Laboratories-I, Ida, Nakahara, Kawasaki, 211 Japan. TOMOHIKO SAKAI is Manager, Nippon Steel Corporation, Hirohata Works, Hirohata, Himeji, 671-11 Japan. YOZO SUGA is Senior Researcher, Nippon Steel Corporation, R& D Laboratories-III, Edamitsu, Yawata-higashi, Kitakyushu, 805 Japan. This paper is based on a presentation made at the symposium "Physical Metallurgy of Electrical Steels" held at the 1985 annual AIME meeting in New York on February 24-28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee. METALLURGICAL TRANSACTIONS A
In a previous paper, 2 the authors have discussed the contribution of the microstructural inhomogeneities which are formed in the hot-rolled sheet in accordance with the different history of phase transformation: the volume fraction of austenite amounts to about 30 pct during hot-rolling for a high-permeability grade steel of typical compositions. A combination of large grains and coarse A1N precipitates, as a consequence of retention of the ferrite phase, has been shown to favor recrystallization after cold rolling of colonies of similar orientation, strongly biased to {110}(001) in the interior of the prior large grains, which coalesce into nuclei of secondary recrystallization. The other combination, of small grains and a fine dispersion of A1N and MnS precipitates, as the products of phase transformation from the austenite, acts as an inhibi
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