Development of Cold Rolled Texture and Microstructure in a Hot Band Fe-3%Si Steel.

Abstract

Hot band Fe-3%Si steel (CRGO or cold rolled grain oriented) was cold rolled with different reductions. The main objective of this study was an overall understanding of deformation texture and microstructure development. Hot band CRGO had a strong a-fiber (RD//͗110͘) texture. Cold reduction strengthened the a and g (ND//͗111͘) fibers, but weakened q (ND//͗100͘). All Taylor type deformation texture models were reasonably successful in predicting these bulk texture developments, and the Lamel model seems to be the 'best-fit' model, both in terms of a 'deviation' parameter (indicating differences between experimental and simulated values of idealized texture components) and a 'trend' parameter (indicating the relative change(s) in texture components with strain). The striking feature of the microstructure was the 'selective' appearance of grain interior strain localization's. These appeared at approximately 37°with the rolling direction (RD). Though 37°bands appeared only in orientations with high Taylor factor (M ), the absolute value of the Taylor factor alone, was not enough for the appearance of such bands. Negative textural softening or (dM/de) values, on the other hand, were always associated with the appearance of 37°bands, justifying or explaining their formation on the basis of a macroscopic plastic instability theory.KEY WORDS: cold rolling; texture; grain oriented electrical steel; microstructure; strain localizations.* 1 However excellent review articles on CRGO are perhaps limited, 4) as compared to forming grade steels. [23][24][25][26] a previous study of the present authors, it was suggested that the relative presence of the respective strain localization's may determine the frequency and 'perfection' of the primary recrystallized Goss grains, perhaps determining the effectiveness of the subsequent secondary recrystallization process. 11) A recent study in low carbon steel 34) has indicated that at least the 37°strain localization's are first generation micro bands forming on the so-called 'high Taylor Factor' orientations. How such strain localization's may evolve in a cold-deformed CRGO and if both 37°and 20°b ands are of the same nature and origin remains to be explored. Experimental MethodsA 200 kg Fe-3%Si steel ingot was hot rolled from 50 to 2.1 mm thickness (pre-heating temperature being 1 330°C). The chemical composition of the hot rolled strip is given in Table 1. The hot rolled strip was annealed at 1 000°C for 60 s and then quenched. The annealed hot-band material was subsequently cold rolled with a laboratory mill to 5 different reductions -25 %, 40 %, 60 %, 77 % and 88 %. Samples were obtained for all these conditions for bulk texture and OIM (Orientation Imaging Microscopy) measurements, using respectively the mid-thickness sections of the rolling plane (containing rolling and transverse directions, i.e. RD and TD) and the long transverse plane (containing rolling and normal directions, i.e. RD and ND).Bulk texture measurements were analyzed by inversion of 4 incomplete pole figures a...

Orientation Dependent Recovery in Interstitial Free Steel

“…A subsequent study 23) indicated that the 20±4° bands actually represent part of a deformed grain at the latter stages of plastic deformation. There appears to be a lot of synergy between the 37±5° bands in CRGO 13,21,23) and the 37° bands 16,22) reported in a typical LC steel. Their preferred appearance, both in CRGO and LC steels, appears to be dominated by suitable instability criterion.…”

“…Conventional microstructures had often revealed 13,19,20) presence of grain interior boundaries in LC steels. In micro-texture studies, these appear 16,21,22) as high angle boundaries inclined at an angle of ~37-40° to rolling direction (RD). Their numbers were reported to be orientation sensitive: 21) a feature often referred as 'preferred appearance'.…”

“…In micro-texture studies, these appear 16,21,22) as high angle boundaries inclined at an angle of ~37-40° to rolling direction (RD). Their numbers were reported to be orientation sensitive: 21) a feature often referred as 'preferred appearance'. These boundaries were referred, in the literature, as microscopic shear bands, 19,20) first generation or non-crystallographic micro-bands 15) and/or more generically as 'grain interior strain localizations'.…”

“…These boundaries were referred, in the literature, as microscopic shear bands, 19,20) first generation or non-crystallographic micro-bands 15) and/or more generically as 'grain interior strain localizations'. 16,[21][22] As the solute content may decide exact deformation and/or recovery processes in LC steels, a generalization of this significant feature of the deformed microstructure as 'grain interior strain localizations' is considered warranted. The present study involves interstitial free or IF steel (less than 60 ppm of carbon and nitrogen converted 2,9) mostly to carbo-nitrides by titanium additions).…”

“…16,18,22) The preferred appearance of strain localizations in deformed γ-fibre grains has long been attributed to higher values of Taylor factor (M) 15,16) and/or stronger textural softening (-dM/dε). 21) This can be shown through a classical instability criterion, such as Dillamore's: 24) ... where σ and ε are the macroscopic stress and strain, n and m are strain hardening exponent and strain rate sensitivity, is the strain rate, M is the Taylor factor and ρm is the mobile dislocation density.…”