Multi-pass Rolling Research Articles

Effects of rolling deformation processes on the properties of Ag-sheathed Sr1−xKxFe2As2 superconducting tapes

The powder-in-tube method is widely used in fabricating iron-based superconducting wires and tapes. To make tapes, a multi-pass rolling process is usually adopted. However, the multi-pass rolling process limits the efficiency of tapes. In this work, rolling deformation technique was studied systematically by fabricating Sr1−xKxFe2As2 superconducting tapes. The total rolling reduction ratio is about 80% and the difference of superconducting performance of tapes rolled by 2, 3, 5 and 7 passes has been investigated. The critical current density Jc, Vickers micro-hardness and microstructure of the superconducting core indicate that tapes after 2, 3, 5 and 7 rolling passes exhibit a similar trend. The width of the tapes and the area of superconducting cores increase with decreasing the number of rolling passes, but the transport Jc of tapes after different rolling passes seems to be the same, except for the tape rolled by 2 passes, whose transport Jc is lower than the other tapes. Concerning the geometry uniformity for the superconducting cores, the sausaging phenomenon was not observed from the photograph of longitudinal cross-section of all the samples. “Lobes” phenomenon on transverse cross-section can be suppressed through decreasing the rolling passes. Therefore, we can obtain uniform and high-performance Ag-sheathed iron-based superconducting tapes by cutting the number of rolling passes down to 3, which is more advantageous to the large-scale producing in the future.

Kinetics of the Static Recrystallization of Austenite in Niobium-Microalloyed Pipe Steels

A method is described for experimentally determining the parameters of a model of the kinetics of static recrystallization. Use of the method ensures a close correlation between calculated and experimental values of the percentage of recrystallized metal (R = 0.97). It is shown that the probability of the occurrence of partial recrystallization between roughing passes must be accounted for in order to evaluate the completeness with which static recrystallization takes place during multipass rolling. It was found that the kinetics of static recrystallization of niobium-microalloyed pipe steels depends on their chemical composition. A convenient software tool, HRRM (Hot Rolling Recrystallization Model), has been created to develop new rough-rolling regimes and correct existing such regimes for the controlled rolling of high-strength microalloyed steels.

Simulation of softening kinetics and microstructural events in aluminum alloy subjected to single and multi-pass rolling operations

In this study, a multi-scale model is proposed to assess softening kinetics and microstructural changes during isothermal annealing within an aluminum alloy. In the first stage, an elastic-plastic finite element analysis is performed for computing the distributions of effective plastic strain and stress while the stored energy after cold rolling is defined based on the predicted data and then utilized for generation of the initial conditions in the microstructural analysis. In the next stage, an algorithm based on cellular automata coupled with a first order rate equation is used to determine the progress of softening behavior at elevated temperatures while both recrystallization and recovery processes are taken into account. The model is examined on single and multi-pass rolling of AA1050 during which the softening progress is measured at temperature varying between 160°C and 360°C. The changes in microstructures and mechanical properties are determined by means of microstructural observations, tensile testing and hardness measurements. Finally, the experimental and the predicted results are compared and a reasonable consistency is observed between the two sets of data indicating the validity of the developed algorithm.

Phase Transformations of the Ti-40% Nb Alloy Under External Influence

The phase transformations of the alloy Ti-40 mas % Nb after tempering and severe plastic deformation are studied. The phase transformations of the alloy according to the type and conditions of external influences are analyzed using methods of XRD, SEM and optical metallography. It is determined that inverse phase transformation of the metastable α''-phase to equilibrium β-phase is carried out after severe plastic deformation. Complete phase transformation α'' → β is typical for the mode, which consists of three pressing operation with the change of the loading axis in cramped conditions, followed by a multi-pass rolling in grooved rolls.

T-bar rolling process with universal and edger mills

Use of T-bars in shipbuilding has increased recently. As T-bars having a web height of at least twice the flange width are generally required in ships, the dimensions of existing hot-rolled T-bars are not suitable. As a result, T-bars for ships are typically fabricated by welding two plates, which is a costly and time-consuming process. To solve this problem, a new rolling method for T-bars using a combination of a universal mill and an edger mill was investigated for the production of T-bars in an actual production line. Finite element analyses of universal and edger rolling were executed to investigate the characteristics of rolling deformation including side camber. Next, multi-pass rolling experiments with a universal roughing mill and an edger mill were performed. Although straight pure lead T-bars were obtained, the web heights at the top and tail ends were larger than those of the other part. To manufacture a product with a constant web height over the whole length, side guide plates were applied in universal rolling and additional vertical rolls were applied in edger rolling. Web height deflection was eliminated by this new facility, and an excellent cross section was obtained over the entire length of the rolled T-bar. The results of this research clearly demonstrated the potential of the new rolling process for producing T-bars for ships.

Thermal stresses and kinetics of phase transformation on the run-out table after hot strip rolling of low-carbon steels

A thermo-elastic analysis coupled with a phase change model is employed to evaluate thermal stresses as well as progress of austenite decomposition after hot strip rolling operations. The thermo-mechanical finite element analysis is utilized to assess the distributions of temperature and thermal stresses during cooling while at the same time, a model based on the additivity rule is used to determine the rate of austenite to ferrite decomposition. The proposed model is applicable to multi-pass rolling schedules while the effects of different process parameters including the rolling speed and cooling configuration of the run-out table can be taken into account. In order to verify the predictions, the measured and predicted temperature-time diagrams are compared and a reasonable agreement is observed. The results show that significant variations occur in the thermal stresses in the period of austenite to ferrite transformation owing to changes in the thermo-physical properties such as the thermal expansion and Young modulus during continuous cooling.

Micromechanical properties of VT1-0 titanium cryorolled to various degrees of strain

Multipass rolling of VT1-0 titanium at a temperature near that of liquid nitrogen to a strain e = −2 resulted in grain refinement from ∼10 μm to ∼35 nm and a nearly twofold increase in microhardness. The microindentation measurements showed that the cryorolled samples had a rather homogeneous structure. An increase in the microhardness with increasing the strain can be described by the modified empirical Voce equation. The grain size dependence of the microhardness in the Hall–Petch coordinates consists of two parts with the slopes kHP1 and kHP2 < kHP1 for the grain size smaller than ∼250 nm. The strong temperature dependence of the microhardness for the investigated samples suggests that their plastic deformation has a thermally activated character. Close values of the thermoactivation parameters presumably indicate a common deformation mechanism in this material placed under an indenter over the entire grain size range.

An upwind least square formulation for free surfaces calculation of viscoplastic steady-state metal forming problems

Despite using very large parallel computers, numerical simulation of some forming processes such as multi-pass rolling, extrusion or wire drawing, need long computation time due to the very large number of time steps required to model the steady regime of the process. The direct calculation of the steady-state, whenever possible, allows reducing by 10–20 the computational effort. However, removing time from the equations introduces another unknown, the steady final shape of the domain. Among possible ways to solve this coupled multi-fields problem, this paper selects a staggered fixed-point algorithm that alternates computation of mechanical fields on a prescribed domain shape with corrections of the domain shape derived from the velocity field and the stationary condition v.n = 0. It focuses on the resolution of the second step in the frame of unstructured 3D meshes, parallel computing with domain partitioning, and complex shapes with strong contact restraints. To insure these constraints a global finite elements formulation is used. The weak formulation based on a Galerkin method of the v.n = 0 equation is found to diverge in severe tests cases. The least squares formulation experiences problems in the presence of contact restraints, upwinding being shown necessary. A new upwind least squares formulation is proposed and evaluated first on analytical solutions. Contact being a key issue in forming processes, and even more with steady formulations, a special emphasis is given to the coupling of contact equations between the two problems of the staggered algorithm, the thermo-mechanical and free surface problems. The new formulation and algorithm is finally applied to two complex actual metal forming problems of rolling. Its accuracy and robustness with respect to the shape initialization of the staggered algorithm is discussed, and its efficiency is compared to non-steady simulations.

Formation Mechanism of SiO2-Type Inclusions in Si-Mn-Killed Steel Wires Containing Limited Aluminum Content

The origin, formation mechanism, and evolution of SiO2-type inclusions in Si-Mn-killed steel wires were studied by pilot trials with systematical samplings at the refining ladle, casting tundish, as-cast bloom, reheated bloom, and hot-rolled rods. It was found that the inclusions in tundish were well controlled in the low melting point region. By contrast, MnO-SiO2-Al2O3 inclusions in the as-cast bloom were with compositions located in the primary region of SiO2, and most CaO-SiO2-Al2O3-MnO inclusions lied in primary phase region of anorthite. Therefore, precipitation of SiO2 particles in MnO-SiO2-Al2O3 inclusions can be easier than in CaO-SiO2-Al2O3-MnO inclusions to form dual-phase inclusions in the as-cast bloom. Thermodynamic calculation by the software FactSage 6.4 (CRCT-ThermFact Inc., Montreal, Canada) showed that mass transfer between liquid steel and inclusions resulted in the rise of SiO2 content in inclusions from tundish to as-cast bloom and accelerated the precipitation of pure SiO2 phase in the formed MnO-SiO2-Al2O3 inclusions. As a result, the inclusions characterized by dual-phase structure of pure SiO2 in MnO-SiO2-Al2O3 matrix were observed in both as-cast and reheated blooms. Moreover, the ratio of such dual-phase SiO2-type inclusions witnessed an obvious increase from 0 to 25.4 pct before and after casting, whereas it changed little during the reheating and rolling. Therefore, it can be reasonably concluded that they were mainly formed during casting. Comparing the evolution of the inclusions composition and morphology in as-cast bloom and rolled products, a formation mechanism of the SiO2-type inclusions in wire rods was proposed, which included (1) precipitation of SiO2 in the formed MnO-SiO2-Al2O3 inclusion during casting and (2) solid-phase separation of the undeformed SiO2 precipitation from its softer MnO-SiO2-Al2O3 matrix during multipass rolling.

A simulation study on the multi-pass rolling bond of 316L/Q345R stainless clad plate

This article describes an investigation into interface bonding research of 316L/Q345R stainless clad plate. A three-dimensional thermal–elastic–plastic model has been established using finite element analysis to model the multi-pass hot rolling process. Results of the model have been compared with those obtained from a rolling experiment of stainless clad plate. The comparisons of temperature and profile of the rolled stainless clad plate have indicated a satisfactory accuracy of finite element analysis simulation. Effects on interface bonding by different parameters including pre-heating temperature, multi-pass thickness reduction rules, rolling speed, covering rate, and different assemble patterns were analyzed systematically. The results show that higher temperature and larger thickness reduction are beneficial to achieve the bonding in vacuum hot rolling process. The critical reduction in the bond at the temperature of 1200 °C is 28%, and the critical thickness reduction reduces by about 2% when the temperature increases by 50 °C during the range from 1000 °C to 1250 °C. And the relationship between the minimum pass number and thickness reduction has been suggested. The results also indicate that large covering rate in the assemble pattern of outer soft and inner hard is beneficial to achieve the bond of stainless clad plate.

Property improvements in fine–grained Mg–Zn–Zr alloy sheets produced by temperature–step–down multi–pass rolling

The microstructure and mechanical properties of Mg–6.63wt.%Zn–0.56wt.%Zr (ZK61) Mg alloy sheets processed by temperature–step–down multi–pass rolling were investigated. A uniform fine–grained structure with the average grain size of about 3.5 μm, which is enriched with rod–shaped β1′(MgZn2) and blocky β1′(Mg2Zn3) precipitates, is attained by multiple dynamical recrystallization. An intense {0002} basal texture with a–axis randomly rotated in the plane is developed. As a consequence, a yield strength of 219 MPa, an ultimate tensile strength of 313 MPa, and a fracture elongation larger than 32% can be obtained. This enhancement in strength during rolling is mainly due to grain refinement and basal texture strengthening in the early stage, and is further contributed by precipitation hardening, simultaneously. Ductility improvement is attributed to the enhanced uniform elongation correlated with twinning suppression, and the enhanced post–necking elongation controlled by grain refinement, especially at the fine grain sizes. This can provide an essential reference for designing high-performance wrought Mg alloy sheets suitable for industrial fabrication.

Achieving higher strength in Cu–Ag–Zr alloy by warm/hot rolling

High-strength Cu–3Ag–0.5Zr alloy plates were produced by multi-pass rolling in the temperature range of 500–800 °C. An increase in strength was observed by rolling in the aforementioned range without significant loss in ductility. All the rolled samples show higher strength than solution-treated and aged samples. The maximum strength was observed for plates rolled at 500 °C with a yield strength and ultimate tensile strength of 311 and 385 MPa, respectively, and retaining a ductility of 23 %. Transmission electron microscopy (TEM) studies showed uniform distribution of fine silver precipitates and high dislocation density in the rolled samples. Nevertheless, the size of precipitates and dislocation density varied with the rolling temperature. The superior strength achieved in the rolled samples is attributed to grain refinement, dislocation strengthening, and precipitation hardening. This method can be employed to produce high-strength plates of precipitation hardenable copper alloys.

Texture evolution of AZ31 magnesium alloy sheets during warm rolling

The purpose of this study was to examine the microstructure and texture evolution of a twin roll cast AZ31 magnesium alloy after multi-pass warm rolling in a temperature range from 233 to 248°C. Reduction per pass was kept constant at about 10% and the total reduction was 98.2% after 33 pass steps with a change of sheet thickness from initially 2.85mm down to 50μm. The rolled sheets exhibited a fairly strong basal texture with an intensity of 17–21 multiples of random distribution (MRD) in the initial stages (with a thickness above 0.47mm) due to the lower rolling temperatures. The basal texture intensity decreased to 13–15 MRD in the later stages (with a thickness below 0.3mm) due to the occurrence of dynamic recrystallization at slightly higher rolling temperatures to balance out the demand of rolling force. The basal pole in the thicker sheets spread slightly along ±TD with an ellipse shape mainly due to the activation of 〈a〉 prismatic slip and the occurrence of shear bands, while the splitting of basal pole in the thinner sheets was observed to be along the RD with a tilting angle of about ±10°. This was attributed to the activation of 〈a+c〉 pyramidal slip. The average grain size in the thinner sheets in the later stages during the multi-pass rolling decreased almost linearly with increasing number of rolling steps. This could be explained via a change in the Zener–Hollomon parameter which was associated with the strain rate affected by the sheet thickness during rolling at a constant percent reduction.

Quantifying texture evolution during hot rolling of AZ31 Twin Roll Cast strip

Multi-pass rolling experiments with an AZ31 Twin Roll Cast (TRC) alloy were performed on an industrial scaled four-high rolling mill. Within the rolling with an intermediate annealing the evolution of texture was investigated. To quantify the extent of preferred crystallographic orientation experimental X-ray pole figures were measured after different process steps and analyzed using the free and open Matlab® toolbox MTEX for texture analysis. The development of the fiber texture was observed and analyzed in dependence on rolling conditions. In the initial state the specimen exhibits a texture composed of a weak basal texture and a cast texture with {0001}-planes oriented across the rolling direction. During the following rolling process a fiber texture was developed. The expected strength increment of the fiber texture was quantitatively confirmed in terms of volume portions of the orientation density function around the fiber and in terms of the canonical parameters of fitted pseudo Bingham distributions. On the results of this work a model for prediction of the texture evolution during the strip rolling of magnesium in the examined parameter range was developed.

FORMING OF BIOINERT ULTRAFINE-GRAINED ALLOYS

The paper presents the results of a research of microstructure and mechanical properties of bioinert alloys based on titanium, zirconium and niobium in ultrafi ne-grained condition. Ultrafi ne-grained condition was received by a combined method of severe plastic deformation, which included high-cycle abc-pressing at a given temperature regime, a multi-pass rolling in shaped rolls at room temperature and low temperature prerecrystallization annealing. The annealing increased the plasticity of alloys in ultrafi ne-grained condition without changing the grain size. In the two-stage severe plastic deformation and annealing the ultrafi ne-grained structure was formed in the alloys. The average element size of the grain-subgrain structure was 0.16 – 0.25 μm, which provided a signifi cant improvement in mechanical properties (ultimate strength, yield strength and microhardness) of the alloys compared to their original hard-grained or fi ne-crystalline conditions. At the same time, the formation of ultrafi ne-grained conditions in alloys did not lead to any change in the elastic modulus with a signifi cant increase in their strength and plasticity.

Production of ultrafine-grain bioinert alloys

The microstructure and mechanical properties of bioinert titanium, zirconium, and niobium alloys in the ultrafine-grain state are investigated. The ultrafine-grain structure is obtained by severe plastic deformation, including multicyclic abc pressing at specified temperatures, multipass rolling in shaped rollers at room temperature, and low-temperature non-recrystallizing annealing. Annealing increases the plasticity of the ultrafine-grain alloys, without changing the grain size. As a result of two-stage treatment—severe plastic deformation and annealing—ultrafine-grain structure with grains and subgrains of mean size 0.16–0.25 μm is formed. That considerably improves the mechanical properties (ultimate strength, yield point, and microhardness) of the alloys. At the same time, the formation of ultrafine-grain structure in the alloys does not change the elastic modulus, even with considerable increase in the ultimate strength and plasticity.

Effect of cryo-deformation on structure and properties of CoCrFeNiMn high-entropy alloy

Microstructure evolution in high-entropy alloy CoCrFeNiMn during plane-strain multipass rolling to a thickness strain of 80% at 293 and 77 K was studied. Deformation at both temperatures was found to be accompanied by twinning. At 77 K, twinning was more extensive in terms of the fraction of twinned grains and the length of the twinning stage thereby providing faster kinetics of the microstructure evolution. Micro-shear bands formed in the microstructure of the alloy at the late stages of rolling (at ε ≈ 80% at 293 K and ε ≈ 40% at 77 K). The ultimate tensile strength of specimens rolled at 77 K or 293 K was found to be 1500 or 1200 MPa, respectively while the strength in the initial homogenized condition was 440 MPa. The contribution of various mechanisms to the hardening of the alloy following rolling at 77 K and 293 K was analyzed quantitatively.

Prediction of microstructure evolution during multi-stand shape rolling of nickel-base superalloys

AbstractIn this paper, a comprehensive numerical approach to predict the microstructure of nickel-base superalloys during multi-stand shape rolling is presented. This approach takes into account the severe deformation that occurs during each pass and also the possible reheating between passes. In predicting the grain size at the end of the rolling process, microstructural events such as dynamic recrystallization (DRX), metadynamic recrystallization (MDRX), and static grain growth are captured at every deformation step for superalloys. Empirical relationships between the average grain size from various microstructural processes and the macroscopic variables such as temperature (T) and effective strain ( ε ̄ ) and strain rate ( ε ̄ ̇ ) form the basis for the current work. These empirical relationships are based on Avrami equations. The macroscopic variables are calculated using a finite element analysis package wherein the material being rolled is modeled as a non-Newtonian fluid with viscosity that depends on the effective strain rate, strain, and temperature. A two-dimensional transient thermal analysis is carried out between passes that can capture the MDRX and/or static grain growth during the microstructural evolution. The presented microstructure prediction algorithm continuously updates two families of grains, namely, the recrystallized family and strained family at the start of deformation in any given pass. In addition, the algorithm calculates various subgroups within these two families at every deformation step within a pass. As the material undergoes deformation between the rolls, recrystallization equations are invoked depending on critical strain and strain rate conditions that are characteristics of superalloys. This approach predicts the microstructural evolution based on recrystallization kinetics and static grain growth only. The methodology was successfully applied to predict the microstructure evolution during the multi-pass rolling of nickel-base superalloys. The predicted results for Alloy 718 for a 4-stand rolling followed by air cooling and for a 16-stand rolling followed by a combination of air and water cooling are also compared with experimental observations.

The microstructure evolution in AZ31B alloy during multi-pass cold rolling to a high reduction

The evolution of deformed microstructure of AZ31B magnesium alloy was investigated during cold rolling through multiple passes. The results show that the AZ31B plate, if appropriately trimmed, can be cold rolled to a high reduction of 95.3%. In this process, twinning plays an important role below 20.7% of the total rolling reduction, and the extension twinning is basically responsible for the formation of the basal texture. At high total rolling reduction, multi-slip is a predominant deformation mode to result in the refinement of the microstructure. The high rollability was explained in terms of multiple slips under multi-pass rolling with small ratio of the plate thickness to roller radius.

Preparation Technology and Mechanical Properties of Al-6Zn-2Mg-2Cu Aluminum Alloy

The combination of atomization and smelting fabricated high strength Al-6Zn-2Mg-2Cu alloy. Microstructure was observed and analyzed by optical microscopy and SEM. The alloy featured with refined grains and even distribution. After being solution treated at 475°C for 1.5h, following multi-pass rolling with total reduction of 33% and aging at 120°C for 40h, the yield strength was 592MPa, peak stress of 597MPa and elongation of 12.2%.