Sharif Digital Repository

In this research, low-carbon steel sheets with two different microstructures containing carbides (as-received steel) and pearlites (austenitized steel) are subjected to severe plastic deformation (SPD), and ultra-rapid annealing (URA) is applied with the heating rate of 200 °C s−1 up to the intercritical temperature near Ac1. Microstructure of SPDed as-received steel after URA is consisted of fully refined ferrite grains surrounded by fine pearlites; but for SPDed austenitized steel, grain refinement is not observed, and microstructure is consisted of martensite and high fraction of coarse non-recrystallized ferrite. Presence of carbides for SPDed as-received steel leads to strong... 

In this work, static strain aging behavior of an alloy steel containing high amounts of silicon and manganese was examined while the influences of initial microstructure and pre-strain on the aging kinetics were evaluated as well. The rate of strain aging in a low carbon steel was also determined and compared with that occurred in the alloy steel. The rates of static strain aging in the steels were defined at room temperature and at 95 °C by means of double-hit tensile testing and hardness measurements. In addition, three-stage aging experiments at 80 °C were carried out to estimate aging behavior under multi-pass deformation processing. The results showed that in-solution manganese and... 

In this research, an algorithm based on the Q-state Potts model is presented for modeling the austenite to ferrite transformation. In the algorithm, it is possible to exactly track boundary migration of the phase formed during transformation. In the algorithm, effects of changes in chemical free energy, strain free energy and interfacial energies of austenite-austenite, ferrite-ferrite and austenite-ferrite during transformation are considered. From the algorithm, the kinetics of transformation and mean ferrite grain size for different cooling rates are calculated. It is found that there is a good agreement between the calculated and experimental results  

Ultra-rapid annealing (URA) of deformed low-carbon steel can provide refined microstructure. In this research, severely deformed low-carbon steel is ultra-rapidly annealed to subcritical and intercritical temperature ranges at the heating rates of 75-1800°C/s and then rapidly cooled. Also, conventional annealing by the heating rate of 0.3°C/s is performed at the same temperature ranges. The results show that conventional and subcritical ultra-rapid annealing cannot lead to a fully refined microstructure. High heating rate causes the interaction between recrystallization and transformation as the temperature exceed Ac1 (intercritical temperature range). High kinetics of transformation during... 

In this work, the effect of hot deformation temperature on microstructure and mechanical properties of micro alloyed steel was studied. The results indicated that by decreasing the deformation temperature final microstructure is refined and the volume fraction of grain boundary ferrite is increased and some pearlite is produced. Therefore both the yield strength and ultimate tensile strength is increased, while the toughness is preserved in comparison to a ferritic-pearlitic microstructure. Also a model was developed to relate the deformation condition to the volume fraction of acicular ferrite at mixed microstructure. © 2008 Elsevier Ltd. All rights reserved  

The strain hardening behavior of AISI 4340 steel with dual phase (DP) and triple phase (TP) microstructures with constant ferrite volume fractions of 34% was investigated. The microstructures of specimens were analyzed using an optical microscope. The tensile flow behavior of all the samples was explained using Holloman equation. Results showed that in both DP and TP steels, work hardening takes place in two stages. The onset strain of second stage of work hardening and Holloman equation parameters (work hardening exponent and strength coefficient) in both stages were increased with the increase of martensite volume fraction in TP steels. At first stage of work hardening, ferrite phase... 

Transformation of austenite to ferrite under continuous cooling condition was investigated. The heat conduction problem was managed by finite element method while two-dimensional cellular automata modeling was simultaneously performed to predict the progress of austenite decomposition using a two-step algorithm to reduce surface-to-volume ratio. Continuous cooling experiments on low carbon steel were made and the ferrite structure was determined and compared with the simulation data. The predicted and the experimental results demonstrated an acceptable consistency and the activation energy for ferrite growth was determined as 171 kJ/mole. The rate of ferrite transformation increased under... 

The objective of this work was to study the influence of titanium on API 5L-X70 steel weld metal properties at manganese levels of 1.4 and 2%. The best mechanical properties in the weld series were obtained in two compositions, i.e. 1.92%Mn-0.02%Ti and 1.40%Mn-0.08%Ti. In both groups of welds, acicular ferrite in the microstructure was increased with addition of titanium in the range of 0.02-0.08%. Manganese helped to refine and homogenize weld microstructures. Increased hardenability of the weld due to further addition of titanium or manganese encouraged grain boundary nucleation of bainite with greater frequency than intragranular nucleation of acicular ferrite. Also, the amount of... 

Transformation of austenite to ferrite under continuous cooling condition was investigated. The heat conduction problem was managed by finite element method while two-dimensional cellular automata modeling was simultaneously performed to predict the progress of austenite decomposition using a two-step algorithm to reduce surface-to-volume ratio. Continuous cooling experiments on low carbon steel were made and the ferrite structure was determined and compared with the simulation data. The predicted and the experimental results demonstrated an acceptable consistency and the activation energy for ferrite growth was determined as 171 kJ/mole. The rate of ferrite transformation increased under... 

This paper attempts to study the microstructural stress-strain evolution and strain localization in the ferrite-pearlite steel by high-resolution experimental-numerical integrated testing. Ferrite crystal orientations measured by electron backscatter diffraction (EBSD) were mapped onto precise scanning electron microscopy (SEM) micrographs of ferrite and cementite lamellar morphologies. Furthermore, in-situ SEM tensile testing was employed to map strains during the deformation using digital image correlation (DIC) at high spatial resolutions. Finally, spectral solver-based crystal plasticity (CP) simulations loaded by the local SEM-DIC boundary conditions were performed and compared to the... 

In this research, we investigate the effects of various microstructural features on the behavior of spherical steel sheets, using the crystal plasticity finite element method. The cementite phase ratio, the grain sizes of ferrite and cementite, and the percentage of residual pearlite in the steel structure due to incomplete annealing are the major microstructural parameters studied in this work. A grain generator software has been developed to generate hexagonal ferrite grains as well as circular cementite particles distributed in the ferrite matrix. We use a hard coating with special properties as an intermediate layer around the cementite grains to simulate the contact between ferrite... 

This study investigates the effect of cold rolling prior to inter-critical annealing on microstructure and mechanical properties of ferrite-martensite dual phase steel. Samples were heated to 850 °C for 1 h followed by oil quenching, then the steel sheet were cold rolled by 0%,10%,15% and 20% reduction in thickness. The inter-critical annealing treatment (750 °C, 120min) was performed to generate a ferrite-martensite microstructure. Microstructural studies showed that increasing the applied cold rolling, leading to increase in volume fraction of martensite and decrease in ferrite grain size. Mechanical properties of dual phase steel were measured by tensile, impact and hardness tests.... 

The influence of thermal cycles on the properties of the coarse grained heat affected zone in X80 microalloyed steel has been investigated. The thermal simulated involved heating the X80 steel specimens to the peak temperature of 1400 °C, with different cooling rates. The four-wire tandem submerged arc welding process, with different heat input values, was used to generate a welded microstructure. The martensite/austenite constituent appeared in the microstructure of the heat affected zone region for all the specimens along the prior-austenite grain boundaries and between the bainitic ferrite laths. The blocky-like and stringer martensite/austenite morphology were observed in the heat... 

In this study a severe plastic deformation method called constrained groove pressing (CGP) is used for imposing a high magnitude of strain into the low carbon steel sheets. Microstructural changes during process are examined by X-ray diffraction and optical observations. The grain size evolution during severe plastic deformation is studied using Williamson-Hall analysis on XRD pattern of the deformed samples. In effective strain of 4.64, ferrite grains with a submicron size of 200-300nm are achieved. The results show that constrained groove pressing can effectively refine the coarse-grained structure to an ultra fine grain range. Mechanical properties changes due to microstructure evolution...