Heavy Cold Rolling Research Articles

MICROSTRUCTURAL CHARACTERISATION OF NON-MAGNETIC Ni-BASED BIAXIALLY TEXTURED SUBSTRATES FOR HTS COATED CONDUCTOR APPLICATIONS

For the production of HTS coated conductor devices, NiV and NiCr (Ni-based alloys) are the most important, non-magnetic, high strength and biaxially textured substrates developed from pure Ni. The Ni 88 V 12 and Ni 85 Cr 15 (at. %) alloys have been melted in Plasma Arc Furnace and textured tapes have been prepared, after heavy cold rolling, by recrystallization heat treatment under high vacuum. The suitable working conditions have been found to obtain not only the {100} <001> cube texture but also the correct grain shape and size for the following deposition process. Out of plane orientation of these substrates has been investigated by θ-2θ X-ray diffraction measurements and in plane orientation study has been completed by pole figures. The microstructure has been observed by optical microscopy: it has been carried out on samples obtained after an annealing treatment (Grain Size Adjustment) and on recrystallized samples in different conditions. As preliminary mechanical investigation the microhardness values have been detected for each step of the working procedure. The accuracy of the cubic texture and the grain structure are affected by the processing condition, in particular the temperature of the GSA seems a very important parameter which influences the final characteristics of the tapes.

PREPARATION AND CHARACTERIZATION OF BIAXIALLY TEXTURED METALLIC SUBSTRATES FOR HTS COATED CONDUCTORS

CuNi, NiCr, NiV alloys and pure Ni have been processed to obtain metallic substrates for high critical current density YBCO film depositions. High purity alloys were prepared using plasma arc furnace and the material was cold rolled to low thickness tapes. For all the tapes the (100)[001] cube alignment was induced by a primary recrystallization heat treatment after heavy cold rolling (RABITS). Besides the cube texture a good substrate should be smooth, thin, strong, non magnetic, resistant to oxidation, cold workable and low cost. In this work mechanical, magnetic and oxidation resistance properties of the prepared textured substrates were measured and compared.

Deformation Microstructure and Tensile Strength of Cold Rolled Pearlitic Steel Sheets

The deformation microstructure produced by heavy cold rolling (from 70 % to 95 % reduction) of pearlitic structure with various amounts of rolling and the tensile strength of the cold rolled sheets were studied in the plain carbon steels with various carbon contents (0.6, 0.76, and 1.0 mass%C). The deformation microstructure was classified into the following three types; 1) Irregularly Bent Lamella (IBL): lamellae originally inclined with large angles to the rolling plane and irregularly bent after deformation, 2) Coarse Lamella with Shear band (CLS) : the rhomboidal blocks of weakly deformed lamellae bounded by shear band, 3) Fine Lamella (FL): heavily deformed lamellae aligned parallel to the rolling direction with fine interlamellar spacings. As rolling reduction increases, the proportion of FL increases.The tensile strength of cold rolled specimens increases with rolling reduction. Work hardening with respect to true strain by cold rolling is similar to that obtained by wire drawing. After 92 % cold rolling of 1.0 mass%C steel, the tensile strength over 2 500 MPa was achieved.

Superplasticity of (α+γ) Duplex Stainless Steel with Elongated Secondary Phase

Effect of initial (α+γ) microstructure on the superplasticity of a hot forged Fe-26Cr-8Ni (α+γ) duplex stainless steel with elongated secondary γ precipitates was examined. The hot forged specimens contain the (α+γ) duplex structure with γ phase elongated along the longitudinal direction of the forged bar. Heavy cold rolling of hot forged specimens promotes recrystallization of γ and α during heating to and holding at 1273K (tensile deformation temperature), resulting in the formation of finer, more equiaxed (α+γ) microduplex structures. The specimens heavily cold rolled after forging exhibit large superplastic elongation nearly the same as the (α+γ) microduplex specimen produced by a conventional thermomechanical processing (solution treatment + heavy cold rolling + aging) although hot forged specimens show much smaller elongation.

Microstructure Development and Superplasticity in (α+γ) Microduplex Alloys with Different Matrix Phases

Formation process and superplastic deformation of (α+γ) microduplex structure by thermomechanical processing (solution treatment+heavy cold rolling+aging) have been studied in the cases for different matrix phases. In Ni-40Cr-6Fe-2Ti-1Al, in which γ (fcc) is matrix and α (bcc) is the secondary phase, α precipitates from deformed γ matrix with a volume fraction of 14% at 1273 K. Subsequently recrystallization of γ takes place and the (γ+α) microduplex structure with high angle boundaries is formed. In Fe-26Cr-Ni alloys, in which α is matrix and γ is the secondary phase, α matrix recovers and subgrain structure with low angle boundaries is formed. Subsequently, γ phase precipitates at α subgrain boundaries, resulting in the (α+γ) microduplex structure. The (γ+α) microduplex specimens of Ni-40Cr-6Fe-2Ti-1Al exhibit superior superplasticity at 1273 K even at a high strain rate of 1.7 × 10 -1 s - because most of grain boundaries are of high angle despite of small α volume fraction. Contrarily, higher γ fractions (40∼50%) is necessary for the appearance of superplasticity in the (α+γ) microduplex structure of Fe-26Cr-Ni in order to suppress a subgrain growth and to promote the transition of α/α (low-angled) and α/γ (coherent) boundary structures to highly angled ones suitable for boundary sliding through dynamic continuous recrystallization of α matrix during tensile deformation.

The effect of the microstructure on the superplasticity of a duplex stainless steel

The aim of this work is to study the effect of the microstructure on the superplasticity of 22% Cr–5% Ni–3% Mo–0.18% N ferrite ( α)–austenite ( γ) duplex stainless steel. To obtain different microstructures several thermo-mechanical treatments were employed. Superplasticity was evaluated at 1253 K by m-value measurements, elongation to failure, deformation homogeneity, and changes in microstructure after tensile tests. A maximum elongation of 1510% was reached with an initial strain rate of 3×10 −4/s. For this condition, the m-value attained 0.64 and necking was not verified. Higher degrees of superplasticity are obtained when the initial microstructure of the material is fine and homogeneous. Such microstructure can be obtained by a solution treatment in the α-single phase field, followed by a heavy cold rolling operation and a new solution treatment in the α/ γ dual phase field. The precipitation of σ-phase during superplastic deformation promotes cavitation and decreases the deformation homogeneity and the elongation to failure.

Recrystallization and Related Phenomena. The Influence of Cold Rolling Reduction on r-value and Recrystallization Behavior in Fe-36Ni Alloy.

Recrystallization behavior and grain growth in Fe-36%Ni invar alloy for shadow masks in color cathode ray tubes are studied by using transmission electron microscopy. The r-value increases with increasing (111) surface intensity. In case of light cold-rolling reduction, nuclei orientations of recrystallized structure after annealing are (011) and the nuclei are surrounded by cold rolled matrices of (011) orientation. On the other hand, in case of heavy cold rolling reduction, nuclei orientations of recrystallized structure after annealing are (011) and (100). These two nuclei are surrounded by the same cold rolled matrices of (011) orientation with light cold rolling reduction.

Correlation of microstructure and type II superconductivity of a heavily cold rolled Cu-20mass% Nb in situ composite

A fibre reinforced in situ metal matrix composite (MMC) consisting of Cu and 20mass% Nb was manufactured by casting and heavy cold rolling without intermediate annealing. The deformation microstructure of the MMC was studied by the use of optical and electron microscopy. The normal and type II superconducting properties of the MMC sheets in the presence of external magnetic fields were examined and compared to the electromagnetic properties of pure Cu and Nb sheets. The results are interpreted in terms of the microstructural changes during rolling deformation. The current findings confirm that the number of internal boundaries and both the filament thickness and spacing have considerable influence on the type II superconducting properties of cold rolled Cu20mass% Nb.

Instability of Secondary Recrystallization Caused by Excessive Cold-Rolling Reduction in Fe-3%Si Alloy

Secondary recrystallization of an Fe-3%Si alloy containing AIN and MnS as inhibitors with heavy cold rolling before primary recrystallization annealing was investigated with special reference to poor secondary recrystallization. Excessive cold-rolling reduction produces smaller primary recrystallized grains with higher intensity of {100} oriented grains and higher possibility for {100} oriented grains associated with Σ5 coincidence boundaries. Specific grain growth of the {100} primary recrystallized grain before the onset of secondary recrystallization is considered to cause the poor secondary recrystallization by reducing the driving force of secondary recrystallization. It may be essential for this unstable secondary recrystallization that the two equivalent orientations of the {100} oriented grains which increase with increasing cold-rolling reduction have the Σ5 coincidence orientation relationship with each other.

Fe-3%Si合金の高冷延圧下率での2次再結晶不安定化機構

Secondary recrystallization of an Fe-3%Si alloy containing AIN and MnS as inhibitors with heavy cold rolling before primary recrystallization annealing was investigated with special reference to poor secondary recrystallization. Excessive cold-rolling reduction produces smaller primary recrystallized grains with higher intensity of {100} oriented grains and higher possibility for {100} oriented grains associated with Σ5 coincidence boundaries. Specific grain growth of the {100} primary recrystallized grain before the onset of secondary recrystallization is considered to cause the poor secondary recrystallization by reducing the driving force of secondary recrystallization. It may be essential for this unstable secondary recrystallization that the two equivalent orientations of the {100} oriented grains which increase with increasing cold-rolling reduction have the Σ5 coincidence orientation relationship with each other.

（α＋γ）二相ステンレス鋼における超塑性変形中の均一微細組織の生成とキャビティの生成挙動

Superplastic deformation behaviors in an (α+γ) duplex stainless steel have been studied with particular emphasis on microstructural change and cavity formation during tensile deformation at 950°C.The main results obtained are as follows;(1) Homogeneous (α+γ) microduplex structure with very fine α and γ grains can be obtained during tensile testing at 950°C at a strain rate of 1.0×10-2/S in the specimens which were solution-treated in the α single phase region and subsequent heavily cold-rolled. The initial cold rolling after solution treatment can play an important role for the formation of homogeneous (α+γ) microduplex structure, that is, the heavy cold rolling promotes the recrystallization of α matrix during hot deformation, resulting in the homogeneous (α+γ) microduplex structure.(2) Superplasticity is considerably enhanced by the prior heavy cold rolling, that is, the maximum elongation at a strain rate of 1.0×10-2S-1 in 90% cold-rolled specimens was 930%. The improvement of superplasticity arises from grain refinement of α and γ during hot deformation.(3) Two types of cavities were observed during hot deformation, that is, the one initiated in the early stage of deformation at the inclusions which were elongated by the prior heavy cold rolling, and the other initiated at grain boundaries and interfaces after the intermediate stage of deformation, suggesting that these cavities especially the former type of large ones may have deleterious effect on the strength and ductility during room temperature service.

A study of grain boundary translational states in a Σ3[ [formula omitted]10]/( [formula omitted]1) bicrystal

High-resolution electron microscopy images of a Σ3[ 1 10]/( 11 1) bicrystal produced by heavy cold rolling and annealing show a multiplicity of grain boundary core structures. Two grain boundary states are separated by a partial grain boundary dislocation with Burgers vector b = 1 2 b 1 (where b 1 = 1 6 [112] is the Burgers vector of a secondary dislocation). This implies a change in translational state. All other structures are related through continuous transport of atoms from one grain to the other resulting in [ 11 1] grain boundary steps. Calculations of the energy associated with the observed structures indicate that the two perfect twin states t = 0 and 2 b 1 are stable, while all other grain boundary translational states are metastable.

High strain rate superplasticity of a 25 Wt Pct Cr-7 Wt Pct Ni-3 Wt Pct Mo-0.14 Wt Pct N duplex stainless steel

Effects of prior thermomechanical treatments on the superplasticity of a 25 wt pct Cr-7 wt pct Ni-3 wt pct Mo-0.14 wt pct N δ/γ duplex stainless steel have been studied by means of hot tensile testing with constant crosshead speeds. The objective is to increase the strain rate suitable for superplasticity. The strain rate is found to be markedly increased by a special prior treatment,i.e., solution treatment at temperatures in the δ single-phase region with subsequent heavy cold-rolling. In hot tensile tests at 1273 K, elongations greater than 1000 and 300 pct were observed at initial strain rates (έ) of 10−3 to 10−1 s−1 and 1 x 100 s−1, respectively. The results for strain rates 〈10−1 s−1 can be explained in terms of a structural superplastic effect due to grain refinement. In the case of έ 〉 10−1 s−1, transformation superplastic effects due to γ-phase precipitation from the σ-ferrite matrix are also important, especially in the early stages of deformation. In the equiaxedδ/γ microduplex structures during stable superplastic deformation, there exists a mixture of two different structures,i.e., dislocated and recovered/ recrystallized δ grains with a homogeneous dispersion of dislocation-free γ particles. This result shows that dynamic recrystallization ofδ grains occurs locally and intermittently due to the dispersion of relatively hardγ particles. The apparent average grain growth rate during deformation is small compared to static grain growth, because grain refinement due to dynamic recrystallization reduces the superplasticity-enhanced grain growth.

Fabrication of thin-film superconductors by bulk processing

A simple processing technique for preparation of c-axis-oriented Bi-Pb-Sr-Ca-Cu-O thin films is reported. ∼3000-Å-thick superconductor films on silver substrate have been obtained by heavy cold rolling and partial melt processing of the superconductor-silver composite structure. The films are highly textured due to the interface-induced, layer-like growth of the superconducting phase, and are essentially free of weak links. They exhibited Tc∼80 K and Jc (4.2 K) ∼104–105 A/cm2. This technique, which does not require epitaxial growth from single-crystal substrates, may be suitable for high-speed, low-cost production of large-area thin films, and points to other novel thin-film processing possibilities.

Amorphization kinetics during cold rolling of MgNi multilayers

MgNi multilayers have been amorphized by cold rolling. Saturation magnetization measurements and X-ray diffraction have been used to monitor the growth of amorphous interlayers of constant composition between the magnesium and nickel layers. The kinetics of amorphization follow the square-root (n) 1 2 of the number of rolling passes, which implies that amorphization scales with the square-root of total rolling time. As such, it can be said to scale with (2Dt) 1 2 where D is the dynamic atomic diffusion constant during thermomechanical treatment time t. It appears that the present results are consistent only with nickel layers that avoid deformation by sliding in the softer magnesium matrix during heavy cold rolling. We have previously observed such deformation behaviour in multilayers whose constituents present strongly different aptitudes for plastic deformation near room temperature.

準安定オーステナイト系ステンレス鋼におけるせん断型逆変態オーステナイトの細粒化機構に及ぼす前加工の影響

Effect of prior deformation on the martensite shear reversion mechanism of the lath martensite to austenite and the grain refining of the reversed austenite has been investigated in a metastable austenitic 14.25%Cr-10.94%Ni steel in terms of the microstructures. The steel used transforms to almost fully lath martensitic structure through 60% cold rolling at room temperature. Further cold rolling above 60% results in an additional deformation to the lath martensite. Martensite lath structures are significantly damaged on and around the slip bands and dislocation cells are formed in the damaged areas. The volume fraction of damaged areas increases with increasing deformation rate.The lath martensite reverts to austenite abruptly with a martensitic shear between 800 K and 900 K, independing on prior deformation. The reversed austenite in the undamaged areas receives the microstructural characteristics of martensite after the reversion. Lath austenitic structure, which is constructed by austenitic, laths and blocks, are formed in the undamaged areas and the traces of slip bands exist in the damaged areas of reversed austenite as a line of dislocation cells. In the undamaged areas, the lath austenitic structure recovers very slowly. Dislocation forms cell structure in the austenitic blocks and the cell structure changes to subgrains. In the damaged areas, recovery of the reversed austenite proceeds very fast and fine recrystallized austenite grains are formed along the traces of slip bands in a short time annealing. In order to obtain uniform fine austenite grains, a lot of slip bands should previously be introduced by heavy cold rolling in the lath martensitic structure.

Deformation and annealing textures of copper deformed in plane strain and by rolling

Rolling is considered to result in plane strain deformation under certain conditions. The texture which results from rolling has been modelled under the assumption of plane strain deformation and also with some relaxation of constraints imposed by the plane strain criterion. The texture which results from the heavy cold rolling of copper and aluminum is very pronounced and generally results in the cube texture on annealing. In this paper a comparison between the textures of copper cold rolled and cold deformed by plane strain compression were examined. Of particular interest in the present study is to see whether there is any difference in the formation of cube texture on recrystallization of the specimens deformed by these two methods. Such an investigation has not been reported in the past. The deformation textures of copper channel die (plane strain) deformed to levels of 80%, 90% and 95% were found to be very similar to that of cold-rolled copper. At reductions of 80% and 90%, the channel-die-deformed copper also produced a stronger cube texture on recrystalization than did the rolled copper. However, at 95% reduction the reverse appeared to be the case. The intensity of the texture due to twins of the cube grains was found to increase slightly compared with the dramatic increase in cube intensity with increasing cold reduction for both rolled and channel-die-deformed specimens.

９９．９９％Ａｌの圧延および再結晶集合組織に及ぼす熱間圧延温度の影響

Rolling and annealing textures of 99.99% aluminum hot rolled at 480° to 590°C, cold rolled by 98.6% and annealed for 1h at 400°C were investigated. They depend on hot rolling temperature; the lower the hot rolling temperature the stronger is S-orientation in rolling texture, while the higher the hot rolling temperature, the stronger is cube orientation after annealing. A traceable amount of (100) nuclei still remain in aluminum hot rolled at elevated temperatures even after heavy reduction cold rolling and grow up to primary recrystallizing origins developing the cube texture.

Plastic deformation of invar alloys

Heavy cold-rolling (95% reduction) of a 64Fe36Ni Invar alloy results in a decrease in magnetization and magnetic hyperfine field at the Fe nucleus, and an increase in electrical resistivity and x-ray line width. These changes are explained by a transition of iron atoms from a high volume-high spin electronic configuration to a low volume-low spin configuration during the deformation process.