Caliber Rolling Process Research Articles

Strain and strain rate hardening effects on the macroscopic shear bands and deformation shape of a caliber-rolled wire

The effect of the strain hardening exponent (n) and strain rate sensitivity (m) on the strain distribution and surface profile of a workpiece during the caliber rolling process has been investigated to fully understand the influence of material properties on shape change and strain distribution of the caliber-rolled wire. Twinning-induced plasticity (TWIP) steel with high n and low m values and low carbon ferritic steel with low n and high m values were mainly compared using both experimental test and finite element analysis. The macroscopic shear bands were clearly observed in the caliber-rolled wire, indicating that the deformation is highly inhomogeneous during the caliber rolling process. The homogeneity of the strain distribution in caliber-rolled wire increased with increasing n and m values due to the fast propagation of the plastic region to other areas from the first hardened area in a material. The effect of the n value was higher than that of the m value. Consequently, TWIP steel had weaker macroscopic shear bands compared to plain carbon steel. The length of the maximum spread of caliber-rolled wire in the free surface increased with decreasing n and m values because the dead metal zone can be easily formed with decreasing n and m values. Therefore, the caliber roll design applied to plain carbon steel cannot be applied to TWIP steel because the roundness of caliber-rolled TWIP steel was poor in such a case. A roll gap thus needs to be modified with the n and m values of a material to fabricate the desired shape of a product.

Wrought-procedure memory in caliber rolled Mg-Y-Zn alloy containing LPSO phase

The possibility to control the deformation kink bands morphology was investigated by the caliber rolling process using the Mg-9 at.%Y-6 at.%Zn alloy with a volume fraction of ~100% in a long-periodic stacking ordered (LPSO) phase. Changing the insertion direction of the rolled billet was an effective method to control the area fraction of deformation kink bands formation. When the rolled billet was rotated 180° to the rolling direction in each rolling pass, deformation kink bands formed in not only the coarse-sized but also the fine-sized LPSO phases. These induced deformation kink bands had a higher density than those of the conventional inserted caliber rolled alloy. This is mainly due to the difference in shear direction to the rolled billet in every rolling pass. The rolled alloy with a high area fraction of deformation kink bands showed superior mechanical properties of hardness and strength at room temperature.

Influence of warm caliber rolling on tensile response and high cycle fatigue behavior of hypereutectoid steel

The effects of severe plastic deformation on the microstructural evolution, tensile and high cycle fatigue properties of warm caliber-rolled hypereutectoid steels were investigated. As the reduction in area, associated with amount of plastic deformation of 0, 38, 57 and 81%, increased in caliber-rolled steels, strain accumulation was observed near interfaces between cementite and ferrite in pearlite structures. Initial pearlite structures were changed to spheroidized cementite and submicron-sized polygonal ferrite by dynamic recrystallization according to the warm caliber rolling process. The initial hypereutectoid steel had a yield strength of 375 MPa, tensile strength of 850 MPa, and elongation of 16.8%. As the rolling strain increased, yield and tensile strength increased dramatically to 1381 and 1604 MPa, respectively. The fracture elongation of the 57% reduction in area (RA) sample decreased to 5.4%. However, it is noteworthy that the total elongation of the 81% RA sample increased slightly to 7.1%. The fatigue limit was also enhanced from 450 MPa for initial hypereutectoid steel to 850 MPa for 81% RA without decreasing the fatigue strength ratio (σfatigue limit/σUTS, 0.53). The 81% RA sample had superior fatigue crack propagation resistance and a high tensile strength–elongation combination compared to that of the initial pearlitic hypereutectoid steel.

Microstructure and mechanical properties of caliber rolled Mg–Y–Zn alloys

The microstructural evolution and mechanical properties were investigated using a caliber rolled Mg–5Y-2.5Zn (in at.%) alloy. Deformation kink bands could be induced by the caliber rolling process, which is the same as that of conventional wrought processed (extruded or rolled) Mg–Y–Zn alloys. The number of rolling passes was an influential factor for the microstructure, i.e., the bending angle of the long periodic stacking ordered (LPSO) phase associated with deformation kink bands. The mechanical properties of strength and hardness also depended on the number of rolling passes. These properties were improved with increased number of rolling passes (up to 6 passes). The yield strength and hardness before the rolling process were 260 MPa and ~90 Hv. On the other hands, after caliber rolling, these properties showed over 500 MPa and 129 Hv, respectively, for the alloy with 6 rolling passes. Microstructural observations revealed that such good properties resulted from the existence of deformation kink bands and dense dislocations induced by caliber rolling. Controlling the morphology of the LPSO phase associated with deformation kink bands was found to be an effective strategy; the bending angle of this phase of more than 20° is outstanding for further improving the mechanical properties.

Effect of Annealing Process on Mechanical and Microstructural Properties of Al6061/Cu and Al/6063/Cu Cladding Materials Fabricated by the Caliber-Rolling Process.

Al6061/Cu and Al6063/Cu cladding materials have been produced by the caliber-rolling process at 298 K. The prepared specimen was post-annealed at a temperature of 773 K for various time durations, and the effects of annealing conditions have been investigated. Vickers hardness of Al6061, Al6063 and Cu decreases as annealing time decreases; however, that of the interface between Al alloys and Cu increases with an annealing time of 1 h, and then decreases as annealing time increases further. The growth of a diffusion layer in Al6063/Cu is larger than that in Al6061/Cu, because the activation energy in the Al6063/Cu is lower than that in Al6061/Cu during the annealing process. Cracking of the diffusion layer occurs by annealing for extended periods of time in both materials, which contributes to the decrease in hardness.

Evaluation of process-induced damage based on dynamic recrystallization during hot caliber rolling

The amount of damage induced during hot forming depends not only on the stress state but also on material softening due to dynamic recrystallization (DRX), which can be used to delay or reduce damage initiation. An extension to the Gurson–Tvergaard–Needleman (GTN) model has been proposed recently that includes a new coupled DRX-damage nucleation criterion taking DRX and the stress state into account. In the present paper, a 4-pass caliber rolling process is considered where the bar stock is rotated by 90° after each rolling pass. The extended GTN model is applied to the caliber rolling process to predict the internal damage induced during the rolling process. The simulation results show that the highest void volume fraction (VVF) occurs during the first rolling pass. However, the variations of stress state during each pass assist in reducing the damage in the subsequent passes. The material softening due to increasing DRX in subsequent passes also helps to reduce the void nucleation. The microscopic analysis of the rolled bar confirms the damage distribution predicted by the simulations.

Effect of Rolling Speed on Microstructural and Microtextural Evolution of Nb Tubes during Caliber-Rolling Process

This study investigated the fabrication of Nb tubes via the caliber-rolling process at various rolling speeds from 1.4 m/min to 9.9 m/min at ambient temperature, and the effect of the caliber-rolling speed on the microstructural and microtextural evolution of the Nb tubes. The caliber-rolling process affected the grain refinement when the Nb tube had a higher fraction of low angle grain boundaries. However, the grain size was identical regardless of the rolling speed. The dislocation density of the Nb tubes increased with the caliber-rolling speed according to the Orowan equation. The reduction of intensity for the <111> fiber texture and the development of the <112> fiber texture with the increase of the strain rate are considered to have decreased the internal energy by increasing the fraction of the low-energy Σ3 boundaries.

Control of mille-feuille structure via caliber rolling process

Deformation kink bands, which are induced by wrought process, are well-known to play a role in increasing the mechanical properties of strength and hardness. However, this microstructural feature such as structure and morphology can vary with thermomechanical processes. In this study, we have investigated the possibility to control such microstructures via caliber rolling process using the Mg-Y-Zn alloy. Microstructural observations of the caliber rolled alloys revealed that deformation kink bands formed irrespective for rolling passes and processed temperatures. In addition, the increase in rolling pass brought about a larger bending angle of long periodic sacking ordered structures associated to deformation kink bands. The properties of strength and hardness enhanced up to 4~6 rolling passes and then they tended to be constant. Good mechanical properties of this caliber rolled alloys were due to existence of both accumulated strains and deformation kink bands.

Effect of strain on mechanical and microstructural properties of Al/Cu claddings during caliber-rolling

Al/Cu claddings were fabricated by caliber-rolling process at 298 K and their mechanical and microstructural properties were evaluated. The Vickers hardness (VH) value of the Al layer in the claddings increase with increasing strain, however, it decreased when the strain was over 0.48. For Cu layer, the VH value rose with strain. The grain size of the Al layer in the cladding decreased with increasing strain and then the grain size increased with a strain of over 0.48. In the case of Cu layer, the grain size increased with increasing strain of the cladding. The relationship between hardness and grain size follows the Hall-Petch equation. During, dynamic recovery is pronounced in the Cu layer, however, grain growth only take place in the Al layer. The evolution of texture is due to a difference in stacking fault energy (SFE) between Cu and Al. The Al layer, with a low SFE, exhibits stronger texture than the Cu layer because of different deformation mechanisms.

Effects of caliber rolling on microstructure and mechanical properties in twinning-induced plasticity (TWIP) steel

The effect of caliber rolling on microstructure and mechanical properties of Fe-Mn-Al-C twinning-induced plasticity (TWIP) steel has been investigated to find alternative methods of wire drawing process using the numerical simulation, electron backscatter diffraction (EBSD) techniques, transmission electron microscopy (TEM), and hardness test. Behavior of twinning, texture, and effective strain was different with areas of rolled wire due to the difference in stress state and strain. The center area had maximum twin density, low angle boundary (LAB), effective strain, and hardness; whereas the surface area had minimum values. In comparison with wire drawing process, caliber rolling process imposed higher stain with slightly uniform distribution on wire, indicating that caliber rolling can manufacture high strength wires more effectively. For instance, after the area reduction of 22% by caliber rolling, the tensile strength was 47% higher and hardness inhomogeneity factor was 15% lower as compared to samples processed by wire drawing. In other words, caliber rolling process was suitable to make high strength materials deformed and hardened by twinning mechanism such as TWIP steels due to the characteristics of imposing severe strain with multi-direction, multi-pass with different shape at each pass, and alternating the loading direction between passes. Therefore, caliber rolling process can be a strong candidate in replacement of wire drawing process, especially TWIP steels.

Effect of Heat Treatment on Microstructures and Mechanical Properties of Severe Plastically Deformed Hypo- and Hyper-Eutectoid Steels by Caliber Rolling Process

This study investigated the effect of post-heat treatment on the microstructures and mechanical properties of severe plastically deformed hypo- and hyper-eutectoid steels that underwent a caliber rolling process. First, 28 passes of caliber rolling were performed on both the hypo-eutectoid steel with Fe-0.47% C (wt%) composition and the hyper-eutectoid steel with Fe-1.02%C (wt%) composition. Then, the caliber rolled materials underwent heat treatment at 500 degrees C for 1, 3, 5, 10, 30 and 60 minutes. The caliber rolled steel possessed a 300-400 nm-sized oval cementite structure created through elongating and segmentation regardless of the C composition. The observation of heat-treated microstructures showed that cementite structure became globular and ferrite size increased as heat treatment temperature increased. In the hardness measurement, the initial caliber rolled samples showed 372.8 Hv (hypoeutectoid) and 480.1 Hv (hyper-eutectoid). However, hardness dramatically decreased up to 10 min. heat treatments, and then showed a constant or small reduction with time. The yield strengths (compression) of caliber rolled hypo- and hypereutectoid steels obtained were 1097 MPa and 1426 MPa, respectively, and the yield strengths of the same steels after heat treatment (500 degrees C, 60 min.) were identified to be 868 MPa and 1316 MPa, respectively.

Manufacturing Ultrafine-Grained Ti-6Al-4V Bulk Rod Using Multi-Pass Caliber-Rolling

Ultrafine-grained (UFG) Ti-6Al-4V alloy has attracted attention from the various industries due to its good mechanical properties. Although severe plastic deformation (SPD) processes can produce such a material, its dimension is generally limited to laboratory scale. The present work utilized the multi-pass caliber-rolling process to fabricate Ti-6Al-4V bulk rod with the equiaxed UFG microstructure. The manufactured alloy mainly consisted of alpha phase and showed the fiber texture with the basal planes parallel to the rolling direction. This rod was large enough to be used in the industry and exhibited comparable tensile properties at room temperature in comparison to SPD-processed Ti-6Al-4V alloys. The material also showed good formability at elevated temperature due to the occurrence of superplasticity. Internal-variable analysis was carried out to measure the contribution of deformation mechanisms at elevated temperatures in the manufactured alloy. This revealed the increasing contribution of phase/grain-boundary sliding at 1073 K, which explained the observed superplasticity.

A study to predict the creation of surface defects on material and suppress them in caliber rolling process

Finite element analysis coupled with a shear fracture model was carried out to predict the creation of surface defects on a material (AISI 4140 steel) and to reduce the surface defects in the caliber (or groove) rolling process, one of steel manufacturing processes. It was assumed that shear fracture owing to shear band localization in the material leads to onset of surface defects of the material during groove rolling. The shear fracture model used to simulate the crashworthiness of aluminum extrusions was applied to the groove rolling after the relation between shear stress ratio and equivalent plastic strain at fracture was modified. To validate the effectiveness of the proposed approach, a roll groove in an actual rod mill (SEAH BESTEEL Inc at Kunsan in Korea) was redesigned and field test was conducted as well. Results revealed that the surface defects on the material during groove rolling could be predicted by checking the number of elements that satisfied the shear fracture criterion. When the newly designed groove was used, finite element simulation of the 6-pass rolling sequence showed that the total number of the damaged elements was reduced from 796 to 629 by 21.0 %. Field test results showed that the total number of products that the surface defects was generated on the surface decreased from 405 to 259 by 36.0 % when the newly designed groove was used.

Microstructure tailoring to enhance strength and ductility in Ti–13Nb–13Zr for biomedical applications

New microstructures were developed by strong grain refinement and phase control in a Ti–13Nb–13Zr alloy. Ultrafine-grained multiphase alloys were fabricated using a multipass caliber-rolling process at the (α + β) region. Transmission electron microscopy analysis revealed that different types of martensitic transformations occurred depending on the cooling rate. The developed alloys exhibited improved strength and ductility compared with a conventional material, and this enhancement of properties is discussed in terms of microstructural effects on the strain-hardening behavior.

Tensile deformation behavior of Fe–Mn–C TWIP steel with ultrafine elongated grain structure

An Fe–17Mn–0.6C steel with ultrafine elongated grain structure was successfully produced utilizing a multi-pass caliber-rolling process at 773K. The uniform elongation of the developed steel was not severely degraded although its strength was greatly enhanced, leading to superior tensile properties. The result contrasted with most ultrafine-grained metals, including C-free TWIP steels, reporting the critical loss of uniform elongation. The peculiar phenomenon in the present work was attributed to the high work hardening capacity of C-added TWIP steels with the submicrocrystalline structure caused by dynamic strain aging, deformation twinning, and formation of stacking faults even after strong grain refinement.

Caliber Rolling Process and Mechanical Properties of High Fe-Containing Al–Mg–Si Alloys

Fe-intermetallic compounds are commonly detrimental for mechanical properties and formability of aluminum alloys. In this study, the refinement of Fe-intermetallic compounds and mechanical properties in high Fe-containing Al–Mg–Si alloys were studied by caliber rolling. The Al–(2.2–2.3) mass%Si–0.9 mass%Mg–(1, 1.5 and 2) mass%Fe alloys were severely deformed by the multi-pass caliber rolling at 573 and 723 K. Fe-intermetallic compounds were finely fragmented with the smallest size of around 200 nm. The morphologies of fragmented Fe compounds in the Al matrix depend on the as-cast microstructure and deformation behavior with better distribution in the outer area than in the center area. Good ultimate strength of 345–360 MPa and high elongation of 15–25% were achieved after 95% caliber rolling.

Process Design Concepts for the Production of Ultrafine Grained Steels through Multi-Pass Warm Rolling: Bridging Science and Technology

Steel bars having a cross section of 18mm square with uniform distribution of ultrafine ferrite grains were produced through a multi-pass warm caliber rolling process in a 0.15%C-0.3%Si-1.5%Mn steel. The average ferrite grain sizes of 0.43μ m, 0.70μ m and 1.2 μ m were obtained in the isothermal warm caliber rolling processes at 773K, 823K and 873K respectively. Even though caliber rolling results in inhomogeneous strain distribution, multi-pass caliber rolling to large cumulated strains of 2 or 3 can be uniformly introduced in to the bar samples. Strain accumulation due to the multi-pass warm deformations was confirmed by comparing microstructural evolution through the multi-pass deformations with that of single pass deformation. The size of ultrafine grains formed through warm deformation was found to depend on the Zener-Hollomon parameter. The similarity of the microstructural evolution with single pass deformation reveals that the multi-pass warm deformation is an effective method to obtain ultrafine grained ferrite structure in bulk materials. It is proposed that compressive strain-Z parameter plots along with grain size-Z parameter plots help in establishing the processing conditions for obtaining products with a desired microstructure and grain size. Finally, such “processing maps” developed for a variety of materials serve useful purpose in bridging the science and technology of developing bulk ultrafine grained materials in semi-finished / finished products.

Optimum Pass Design of Bar Rolling for Producing Bulk Ultrafine-Grained Steel by Numerical Simulation

The groove design for creating ultrafine-grained low-carbon steel through a caliber rolling process was studied from the viewpoint of the large strain accumulation and cross-sectional shape variation in a bar. A three-dimensional finite element analysis was employed for this purpose. The caliber rolling process of foval (flat-like-oval)/square type was proposed as a method to efficiently introduce a large strain in material. The relationship among the foval configuration, strain, and cross-sectional shape was examined in the caliber rolling. The influence of the equivalent strain distribution by 1st pass (foval rolling) depends strongly on the strain distribution and a cross-sectional shape by 2nd pass, and the foval configuration to accumulate a large strain efficiently was shown. The optimum pass schedule to fabricate a 13mm square bar of ultrafine-grained steel from a 24 mm square bar by caliber rolling at warm working temperatures was proposed.

実機溝ロール圧延機による超微細粒組織棒鋼の創成

A caliber-rolling process with new type groove characterized by multidirectional deformation was proposed to fabricate ultrafine-grained (UFG) steel bars efficiently. To impose a large strain into center of cross-section, the groove configuration of foval (flat-like-oval) type was designed. Three-dimensional finite element simulation (FE simulation) was utilized for this purpose, and the caliber-rolling process of square/foval pass was investigated from the viewpoints of a large strain accumulation and cross-sectional shape variation in a bar. The UFG steel bars were fabricated by using the commercial caliber-rolling with the grooves designed from FE simulation. The cross-sectional shape predicted from FE simulation was in excellent agreement with the shape of the rolled bars. The strain imposed in the rolled bars has a distribution with maximum at the center. The giant strain of 5.2 is accumulative in center regardless of reduction in area of 79%. The good correlation between the strain imposed and the hardness of the rolled bars was confirmed.

オーバル孔型とスクエア孔型を用いた多方向大ひずみ圧延によって生成する微細粒組織と力学的性質の関係

Multidirectional warm caliber rolling process using oval grooves and square grooves was developed. Microstructure and mechanical properties obtained by this process in a low carbon steel were investigated. Ultrafine grains with high angle grain boundaries were generated in the center region in a material by just 2-pass oval-square caliber rolling. The region having ultrafine ferrite grains expanded along with rolling and almost all of region was occupied with ultrafine ferrite grains with high angle grain boundaries by 6-pass oval-square caliber rolling. Although apparent rolling reduction in this 6-pass oval-square process is 71%, large plastic strain larger than 3 can be introduced into materials due to the effect of multidirectional deformation. Comparing with square-square caliber rolling in which 21-pass is required to obtain fully ultrafine grained structure, oval-square caliber rolling is very efficient process to create ultrafine microstructure.