The current understanding of the fundamentals of recrystallization is summarized. This includes understanding the as-deformed state. Several aspects of recrystallization are described: nucleation and growth, the development of misorientation during deformation, continuous, dynamic, and geometric dynamic recrystallization, particle effects, and texture. This article is authored by the leading experts in these areas. The subjects are discussed individually and recommendations for further study are listed in the final section.

/pdf/current-issues-in-recrystallization-a-review-12181rj4pn.pdf

Current issues in recrystallization: a review

Crystallographic variant selection in Ti–6Al–4V

During the hot rolling of steels, the parent austenite phase develops a crystallographic texture. As a result, the material after transformation (ferrite, martensite, acicular ferrite, or bainite ) also acquires a texture which is related in a precise way to the texture of the parent material. The major component of the transformation texture derived from recrystallized austenite is the {001} , which originates from the cube {100} component of the parent texture. The major components of the texture of pancaked austenite are the Bs {110} and Cu {112} , and these give rise, respectively, to the {332} and {113} components in the transformation product. The Goss {110} , Goss/Bs {011} , Bs/S {168} , S{123} , and S/Cu {236} orientations in the deformed austenite also contribute to the formation of the overall transformation texture. Since more than one parent γ orientation can transform into the same bcc orientation, it is sometimes impossible to determine, unambiguously, the origin of particular transformation texture components in the parent γ texture. The intensity of the overall transformation texture can be increased by the addition of Nb, Ti and V to steel, by giving large reductions during controlled rolling, and by using lower finishing temperatures during processing. Among the transformation texture components, the {332} is the most beneficial from the point of view of achieving good deep drawability and improved strength and toughness. The presence of substitutional solutes such as Mn, Ni, Cr or Mo, finer austenite grain sizes and faster cooling rates during transformation increase the intensity of the {332} component. By contrast, the intensity of the {113} component remains relatively insensitive to the above factors.

Transformation Textures in Steels

Effects of ausforming temperature on bainite transformation, microstructure and variant selection in nanobainite steel

The mechanisms by which textures can be inherited in transformed phases are discussed in the light of different transformation mechanisms. Possible origins of variant selection in the different cases are reviewed and classified. Evidence is presented for a hitherto unsuspected source of variant selection that arises from the stresses which are generated during transformation due to the presence of micro-segregation. Some model predictions show the potential effect of this phenomenon on textures in bainite or martensite.

The way in which texture development is affected by austenite pancaking was studied in a plain carbon and a niobium steel. Three processing parameters (austenitizing temperature, finish rolling temperature and amount of reduction) were varied, and their influence on the state of the austenite was analyzed. It was found that the presence of niobium, which leads to pancaking of the austenite when low finishing temperatures are employed, strengthened the transformation products produced from the copper, brass, S and Goss orientations in the austenite. Larger reductions resulted in sharper textures, especially in the microalloyed steel. The effect of austenitizing temperature was not particularly strong. Several f.c.c. ideal orientations were transformed according to the Kurdjumov-Sachs relationship, and the b.c.c. products resulting from this analysis are compared with the experimental results. When recrystallized austenite transforms, it is shown that the cube component transforms preferentially into the {001} rotated cube orientation, with lower than expected intensities displayed by the Goss and rotated Goss components. Similar trends are observed with respect to the four main deformation components; these tendencies are interpreted in terms of variant selection and selective growth.

Effect of austenite pancaking on texture formation in a plain carbon and A Nb microalloyed steel

The effect of finish rolling temperature was investigated on texture formation in a plain C and a 0.034% Nb microalloyed steel. When finish rolled at 1020°C (i.e. within the γ recrystallization range), the textures in both steels contain the {001} and {110} components. The sharpness of the {001} component generally increases with decreasing finish rolling temperature down to 630°C, while the {110} component gradually weakens and finally disappears after ferrite rolling. The microalloyed steel displays much sharper texture than the plain C steel when finish rolled at 870°C (i.e. within the γ pancaking range for the Nb steel) and at 730°C (in the γ+α intercritical range). After finish rolling at 870°C, the major texture components in the microalloyed steel are {113} and {332} , in addition to the above two, while the plain C steel texture only contains some low intensity maxima. When finish rolled at 730°C, weak peaks appear at {223} and {554} in the plain C steel and stronger ones at {4411} and {554} in the microalloyed steel. After warm rolling at 630°C, the major texture components in both steels are {223} , {554} and {001} .The {001} and {110} components are obtained, by transformation, from the {100} (cube) and {122} (twinned cube) components of the recrystallized γ. By contrast, the {113} and {332} components originate, respectively, from the {112} (copper) and {110} (brass) components of the unrecrystallized γ. During continued rolling in the γ+α or α range, these transformation texture components are further modified by deformation and ultimately give rise to the stable end orientations which constitute the well-known warm rolling texture in steels.

/pdf/effect-of-controlled-rolling-on-texture-development-in-a-tqrfqix0br.pdf

Effect of controlled rolling on texture development in a plain carbon and a nb microalloyed steel

Samples of a Nb microalloyed steel were control rolled and finished (i) at 1020°C (in the γ recrystallization range) and (ii) at 870°C (in the γ no-recrystallization range). The textures of these materials were compared with b.c.c. transformation textures derived analytically from (i) annealed and (ii) cold rolled samples of an f.c.c. Ni30Co alloy. For this purpose, the Kurdjumov-Sachs relationship was assumed, together with the absence of variant selection. The ferrite texture of the experimental steel finish rolled in the γ recrystallization range is sharper than the corresponding simulated texture, the major components of both being {100}〈011〉 and {110}〈110〉. The texture of the experimental steel finish rolled in the γ no-recrystallization range is again sharper than the corresponding simulated texture, both containing {113}〈110〉, {332}〈113〉 and {100}〈011〉 as the major components. The detailed textural features of the steel finished at 1020°C can be explained by allowing for some variant selection, together with selective growth of the {100}〈011〉 oriented α grains during transformation. A combination of selective α grain growth and variant selection is again required to explain the detailed texture of the steel sample finished at 870°C.

M.P. Butrón-Guillén

Papers

Transformation Textures in Steels

Effect of austenite pancaking on texture formation in a plain carbon and A Nb microalloyed steel

Effect of controlled rolling on texture development in a plain carbon and a nb microalloyed steel

Comparison between simulated and experimental transformation textures in a Nb microalloyed steel

Effect of Austenite Recrystallization on Ferrite Transformation Textures