Recrystallization and Grain Growth

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Authors: M. Draissia, N. Boukhris, M.Y. Debili
Abstract: The thermomechanical treatment at 1050°C under a stress of about 30 MPa, of milled ribbons from Fe-25Cr-20Ni (0.060%Ni-0.1%Ti) refractory stainless steel, leads to a recrystallisation of the as-melt-spun structure which is intermediate between cellular and columnar dendritic. The mean grain size in the relatively high density zones (85%) may be considered as low and do not exceed 10µm. Other grains appear abnormally large and reach 30µm. The origin of these grains, must be researched in an exaggerate growth phenomenon under a local deformation near the critical work hardening.
247
Authors: R. Ebrahimi, A. Oudin, A. Najafi-Zadeh, Matthew R. Barnett, Peter D. Hodgson
Abstract: The present work examines the effect of strain and Zener Hollomon parameter, Z, on deformation and recrystallization of Ti-IF steel deformed in the warm temperature region. Torsion tests were performed at temperatures of 765oC and 850oC and strain rates of 0.003s-1 and 1s-1. For some conditions, an annealing treatment at the temperature of deformation was carried out using a fluidized bed furnace. Electron Back Scatter Diffraction (EBSD) maps were implemented to study the microstructure evolution. At a low value of Z abnormally large grains formed during annealing. At the higher values of Z, a more classical recrystallization reaction occurred during annealing. The latter is more desirable in most commercial applications.
251
Authors: A. Elsner, Radko Kaspar, Dirk Ponge, Dierk Raabe, Sybrand van der Zwaag
Abstract: The ferritic rolling strategy allows for the production of two different hot strip grades, a "soft" and a "hard" hot strip. The "soft" hot strip is rolled in the upper ferrite region and a sufficiently high coiling temperature ensures direct recrystallisation in the coil. The "hard" hot strip is rolled at relatively lower temperatures in the ferrite temperature region and exhibits a strained microstructure with a desirable rolling texture. Furthermore, these ferritic rolled hot strips can be used as initial strip for subsequent cold rolling. The current investigation focuses on the development of the recrystallisation texture of cold rolled and annealed ferritic rolled hot strip for different cold reductions. For this purpose "soft" and "hard" hot strips were produced on a laboratory hot rolling mill. These strips were cold rolled with a total reduction of 40 to 80% to a final thickness of 0.5mm. Subsequently the strips were subjected to simulated continuous annealing, using a salt bath furnace. The macro texture of both types of specimens was measured and correlated to the mechanical properties, including the Lankford values. A very different development of the recrystallisation texture and hence mechanical properties has been observed. However, both grades yielded improved deep-drawing properties.
257
Authors: Y. Futamura, Masahide Natori, Toshihiro Tsuchiyama, Setsuo Takaki
Abstract: Microstructural change and soft ening behavior during annealing were investigated for deformed ferrite and lath martensite in an ultralow carbon 1.5mass%Mn-0.0018mass%B steel, and then the difference in recrystallization behavior between the materials was discussed in terms of the nucleation site of recrystallized grains. The ferritic and martensitic materials were obtained by furnace-cooling and water-quenching, respectively, after solution treatment. The ferritic material was cold-rolled at a reduction of 80% to give the same dislocatio n density as of the martensitic material. The deformed ferritic material contains a large number of geometrically necessary boundaries with large misorientations, while the martensitic material does only contain original grain boundaries such as prior austenite grain boundaries, packet boundaries and block boundaries. The recrystallization during annealing is markedly retarded in the martensitic material compared with the deformed ferritic material. As a result, the time for completing the recrystallization was roughly a hundred times longer in the martensitic material than in the deformed ferritic material. This is due to the difference in nucleation site of recrystallized grains, that is, the geometrically necessary (GN) boundaries introduced by the deformation for the ferritic material, and only the original grain boundaries for the martensitic material.
263
Authors: Hotaka Homma, Shuichi Nakamura, Naoki Yoshinaga
Abstract: Heavily cold rolled BCC steel has been indicated to generate {411}<148> recrystallisation texture and its family orientations which might be represented as {h,1,1}<1/h,1,2>. As a-fibre structure, or RD//<011> texture is significantly developed during the cold rolling, it is naturally speculated to be the recrystallisation site of {h,1,1}<1/h,1,2> fibre. The present paper prompts to demonstrate the recrystallisation procedure by utilising EBSP-OIM analysis. The first demonstration was carried out with OIM analysis on partially recrystallised cold rolled steel. At the stage of 50% recrystallisation, only ND//<111> texture has appeared for the recrystallised area. {100}<011> - {211}<011> a-fibre remains as deformed structure, and several {h,1,1}<1/h,1,2> grains could be found at the grain boundaries. Therefore, a bi-crystal of {100}<011> was employed to simulate the irregular deformation at the grain boundary. After cold rolling, a warp toward the grain boundary was observed. Although the interior of the {100}<011> single crystal was hardly recrystallised, sharp {411}<148> texture was created along the grain boundary. In order to confirm the phenomenon, another experiment was carried out that a cold rolled {100}<011> single crystal was bent along the rolling direction and annealed. Very sharp {411}<148> recrystallisation texture was formed again at the bent perimeter. These experimental results lead us to conclude that the irregular strain was sufficiently piled at the grain boundary after the heavy deformation and generates {h,1,1}<1/h,1,2> texture. On {100} pole figures, the recrystallisation textures were equivalently scattered around three <100> poles, therefore the rotation relationship around <111> axes with the original orientation was suggested.
269
Authors: D. Jorge-Badiola, Amaia Iza-Mendia, Isabel Gutiérrez
Abstract: It has been demonstrated recently that when a reversion of the strain is applied during the hot working of a Nb-microalloyed steel, the subsequent static recrystallization kinetics is significantly affected. However, depending on the magnitude of the reversion, the static softening kinetics can be accelerated or delayed. This relates to the substructure dissolution taking place by the effect of the reversal. In the present work, new microstructural results obtained by EBSD on an austenitic stainless steel hot deformed by torsion is used to explain better the observed effect of the reversion of the strain.
275
Authors: Liudmila M. Kaputkina, Vera Prokoshkina, Yu.I. Lojnikov
Abstract: Kinetics of deformation strengthening, polygonization and recrystallization processes have been studied, effects of alloying by nitrogen, combined carbon and nitrogen as well as by various other elements (Cr, Mo, Ni, Mn, V etc.) have been estimated for steels of different compositions and applications. Strain diagrams and structure state maps for the studied steels are presented. Strain diagram shape and attainable hot strength depend on the deformation conditions and basic alloying which determine strain hardening and diffusional processes of post-deformation softening. Alloying by nitrogen increases hot and cold strain hardening and retards recrystallization. Maximum strengthening obtained by cold deformation is accompanied by lowering of ductility and fracture toughness. Hence, it is applicable mainly to the austenitic steels. Nitrogen alloying enhances the austenite stability against g ® a transformation and consequently allows extending a composition range of steels which can be strengthened by cold deformation with large strains. The high-temperature thermomechanical treatment is more effective as a treatment improving a combination of mechanical properties. The schemes and regimes of thermomechanical strengthening treatments are proposed for low- and high- nitrogen containing steels of various structure classes.
281
Authors: Leo Kestens, Ana Carmen C. Reis, Wlodzimierz Kaluba, Yvan Houbaert
Abstract: A Ti-stabilized interstitial free steel was highly cold deformed to a reduction of 95% and subsequently submitted to extremely short annealing cycles with heating rates varying between 300°C/s and 4500°C/s followed by water quench at various temperatures. The microstructural and textural development was studied through various consecutive stages: partially recrystallized, fully recrystallized and after α→γ→α transformation. It was found that irrespective of the heating rate the recrystallization has completely terminated before the onset of the ferrite to austenite phase transformation. In the fully recrystallized condition, ultra-rapid heating gave rise to substantially refined structures with an average ferrite grain size of 6µm. It was also observed that this grain refinement saturates with heating rates beyond 1000°C/s. With regard to the texture formation, the characteristic {111} deep drawing fibre of cold rolled IF steels was observed, irrespective of the heating rate, in an annealing treatment as short as 0.3s. After the forward and reverse α→γ transformation, the ensuing ferrite texture displayed a strong memory effect, as the {111} fibre was even more intense after the double transformation than before.
287
Authors: Haiwen Luo, Jilt Sietsma, Sybrand van der Zwaag
Abstract: The austenite recrystallization kinetics in the intercritical region of a C-Mn steel is investigated by means of stress relaxation tests. It is found that the Avrami exponent, n, decreases significantly with decreasing temperature, i.e. with increasing ferrite fraction. This behaviour deviates from that of austenite recrystallization in the purely austenitic state, in which case the Avrami exponent is constant and independent of temperature and deformation. To interpret this, the influence of spatial variation of the plastic strain in the intercritical austenite grains on recrystallization kinetics is modelled quantitatively. The modelling results seem to indicate that the strain heterogeneity is responsible for the decreasing Avrami exponent with decreasing intercritical temperature.
293
Authors: Eun Hye Na, Jae Young Choi, Baek-Seok Sung, Hu Chul Lee
Abstract: The effect of carbo-nitride precipitation on the recrystallization behavior of Ti-stabilized extra low carbon steels was investigated. The precipitation behavior of titanium carbo-nitride was analyzed using transmission electron microscopy (TEM), a chemical extraction method, and the small angle neutron scattering (SANS) method. The recrystallization temperature was varied from 590°C to 680°C depending on the alloy chemistry and hot rolling process. The total amount of precipitates in the hot bands did not significantly affect the recrystallization temperature of the alloys. Isothermal annealing treatment showed a plateau in the stress relaxation curves, i.e. a delay in the recovery, when precipitation occurred during the annealing treatment. A model developed to explain the dynamic interaction of the precipitation with the recovery in the microalloyed austenite was successfully adopted to simulate the delay of the recovery during the recrystallization treatment of the cold rolled Ti-stabilized extra low carbon steels.
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