Volumes 783-786

doi: 10.4028/

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Authors: Evgueni I. Poliak, Debanshu Bhattacharya
Abstract: The production of advanced high strength steels (AHSS) has been rapidly expanding in recent years as these steels allow for considerable reduction in weight and enhancement of car safety due to the unique combination of high strength, toughness and formability. Driven by growing demand for sheet AHSS products from carmakers, steel producers are currently developing AHSS of the so called 3rd Generation to further facilitate weight reduction of critical safety parts while ensuring crash worthiness and high absorbed energy. Such steels not only possess tensile strength above 1000 MPa but also are being designed for exceedingly high formability: high elongation, bendability, hole expansion and strain hardening. These enhanced properties are to be achieved in final operations of continuous annealing and/or galvanizing. However, due to complicated alloy designs of 3G AHSS the role of each manufacturing stage becomes progressively significant due to its impact on the final microstructure. Therefore, hot strip rolling gains increasing importance as one of the most critical stages responsible for producing the microstructure optimal for achieving the final properties of the sheet products without impairing downstream operations. In other words, hot rolling of AHSS has to be viewed as thermomechanical processing.
Authors: Subrata Chatterjee, S. Kundu, S. Sam, B. Mishra
Abstract: In the present study, the microstructure and strength properties of diffusion bonded joints of duplex stainless steel (SS) to Ti alloy (TiA) with and without nickel alloy (NiA) as an intermediate material was investigated in the temperature range of 800-950°C for 60 mins in vacuum. In the case of directly bonded stainless steel and titanium alloy, the layer wise σ phase and λ+FeTi phase mixture were observed at the bond interface. However, when nickel alloy was used as an interlayer, the interfaces indicate that Ni3Ti, NiTi and NiTi2 are formed at the nickel alloy-titanium alloy interface and the stainless steel-nickel alloy interface is free from intermetallics up to 875°C and above this temperature, Fe-Ti and Fe-Cr-Ti base intermetallics were formed. The irregular shaped particles have been observed within the Ni3Ti intermetallic layer. The joint tensile and shear strength were measured; a maximum tensile strength of ~519.2MPa and shear strength of ~398.3MPa were obtained for direct bonded joint when processed at 875°C. However, when nickel base alloy was used as an interlayer in the same materials at the bonding temperature of 900°C the bond tensile and shear strength increases to ~596.5MPa and ~434.4MPa, respectively. Keywords: Diffusion bonding, intermetallic compounds, interlayer, SEM, XRD
Authors: Helmut Clemens, Svea Mayer
Abstract: After almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ-TiAl phase have found applications in aircraft and automotive engine industry. The advantages of this class of innovative high-temperature materials are their low density and their good strength and creep properties up to 750°C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi-phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e. concurrent increase of ductility at room temperature and creep strength at elevated temperature.
Authors: Xiao Jun Liang, Ming Jian Hua, Anthony J. DeArdo
Abstract: Thermomechanical controlled processing is a very important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes the control of the processing important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the roles of austenite conditioning and cooling rates on the bainitic phase transformation. Specimens were compared with and without deformation, and followed by seven different cooling rates ranging between 0.5°C/s and 40°C/s. The CCT curves showed that the transformation behaviors and temperatures are very different. The different bainitic microstructures which varied with austenite deformation and cooling rates will be discussed.
Authors: Leszek Adam Dobrzański, Wojciech Borek, Janusz Mazurkiewicz
Abstract: Taking into consideration increased quantity of accessories used in modern cars, decreasing car’s weight can be achieved solely by optimization of sections of sheets used for bearing and reinforcing elements as well as for body panelling parts of a car. Application of sheets with lower thickness requires using sheets with higher mechanical properties, however keeping adequate formability. The goal of structural elements such as frontal frame side members, bumpers and the others is to take over the energy of an impact. Therefore, steels that are used for these parts should be characterized by high value of UTS and UEl, proving the ability of energy absorption. Among the wide variety of recently developed steels, high-manganese austenitic steels with low stacking faulty energy are particularly promising, especially when mechanical twinning occurs. Beneficial combination of high strength and ductile properties of these steels depends on structural processes taking place during cold plastic deformation, which are a derivative of SFE of austenite, dependent, in turn on the chemical composition of steel and deformation temperature. High-manganese austenitic steels in effect of application of proper heat treatment or thermo-mechanical treatment can be characterized by different structure assuring the advantageous connection of strength and plasticity properties. Proper determinant of these properties can be plastic deformation energy supply determined by integral over surface of cold plastic deformation curve. Obtaining of high strength properties with retaining the high plasticity has significant influence for the development of high-manganese steel groups and their significance for the development of materials engineering.
Authors: Hirofumi Inoue
Abstract: A new rolling process, which combined asymmetric rolling with symmetric rolling, was adopted in age-hardenable 6xxx series Al-Mg-Si alloy promising as automotive body panels in order to develop favorable textures for the deep drawability after solution treatment. Symmetric cold rolling at high reduction and subsequent asymmetric warm rolling at low reduction for AA6022 sheets led to the formation of “TD-rotated β-fiber texture” including moderate {111}<uvw>-oriented components, resulting in noticeable evolution of {111}<110> recrystallization texture during the solution treatment at a high temperature. The results of texture analysis and microstructural observation suggested that the low stored energy after asymmetric warm rolling, the high fraction of high angle boundaries with neighboring deformed matrices and the approximate 40° <111> orientation relationship with deformed matrices would strongly affect the evolution of {111}<110> recrystallization texture.
Authors: John J. Jonas, Clodualdo Aranas, Vladimir V. Basabe, Chiradeep Ghosh
Abstract: Seven-pass strip rolling simulations were carried out on a 0.06%C and a 0.09%C-0.036%Nb steel. The rolling loads (mean flow stresses or MFS’s) did not increase as the temperature decreased during the simulation. This is ascribed to the occurrence of dynamic transformation. The simulation results are compared to the high temperature flow curves determined on eight plain C and Nb-modified steels in both compression and torsion and at a series of temperatures and strain rates. When the associated MFS’s are plotted against inverse absolute temperature in the form of Boratto diagrams, the stress drop temperatures, normally defined as the upper critical temperature applicable to rolling, Ar3*, are shown to be about 40 degrees above the paraequilibrium and about 20-30 degrees above the orthoequilibrium Ae3’s. These drops are ascribed to the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance of fine Widmanstätten plates. These plates coalesce into polygonal grains on further deformation and on holding.
Authors: Yi Huang, Megumi Kawasaki, Terence G. Langdon
Abstract: High-pressure torsion (HPT) is a processing technique in which samples are subjected to a high pressure and torsional straining. Experiments show that the shearing patterns in HPT are dependent upon the alignment of the anvils within the HPT facility. Using a two-phase duplex stainless steel as a model material, experiments were conducted by placing the anvils in different amounts of initial misalignment. The results demonstrate the importance of always ensuring that the upper and lower anvils are in good alignment prior to HPT processing.
Authors: Dong Nyung Lee, Heung Nam Han
Abstract: The cold-rolling texture of fcc sheet metals with medium to high stacking fault energies is known to consist of the brass {011}<211>, Cu {112}<111>, Goss {011}<100>, S {123}<634>, and cube {100}<001> components. The recrystallization (Rex) texture of cold-rolled Al, Cu and their alloy sheets is well known to be the cube texture. The 40°<111> orientation relationship between the S and cube components, which has been taken as a proof of the oriented growth theory, has made one believe that the S orientation is responsible for the cube Rex texture. The oriented growth theory is claimed to be associated with grain boundary mobility anisotropy. However, some data indicate the Cu component is linked with the cube component. There is no 40°<111> orientation relationship between the Cu and cube components. The strain-energy-release-maximization model (SERM), in which the strain energy due to dislocations is importantly taken into account, suggests that the Cu and S components in the rolling texture are linked with the cube and ~{031}<100> components in the Rex texture, respectively.

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