فهرست مطالب

iron and steel society of Iran - Volume:15 Issue:2, 2018
  • Volume:15 Issue:2, 2018
  • تاریخ انتشار: 1397/09/03
  • تعداد عناوین: 7
|
  • E. Abbasi *, W. M. Rainforth Pages 1-11

    This paper reports on the precipitation behaviour and microstructural evolution in ~20% cold rolled NbMoV and V single microalloyed low carbon steels during intercritical annealing. The microstructure and precipitation behaviour were studied by optical and scanning/transmission electron microscopy and microanalysis, X-ray diffraction technique and Vickers hardness testing. After intercritical annealing, the resulting microstructure in both steels comprised acicular/bainitic ferrite matrix with an uneven proportion of allotriomorphic ferrite and retained austenite and martensite. The average density of microalloying precipitates increased in both steels during intercritical annealing. NbV, NbMoV and V carbides were observed in the NbMoV steel, while V-carbide extensively appeared in the V steel. The results also showed a much greater precipitation strengthening in the NbMoV steel after intercritical annealing compared to the V steel. The overall findings suggested that NbMo addition could retard the growth/coarsening of microalloy precipitates with a size of lower than 15nm during the intercritical annealing.

    Keywords: Microalloyed steel, Intercritical annealing, Precipitation, Acicular-bainitic ferrite
  • M. Zamani, H. Mirzadeh*, M. Nouroozi Pages 12-17

    The effect of microstructural refinement and intercritical annealing on the mechanical properties and work-hardening response of a low carbon St12 steel was studied. It was revealed that intercritical annealing of the ferritic-pearlitic sheet results in the formation of a coarse-grained DP microstructure with discrete martensite islands normally formed in place of pearlitic colonies, which results in the minor enhancement of mechanical properties with disappearance of the yield-point elongation. On the other hand, a fine-grained DP steel with chain-network martensite morphology can be obtained by intercritical annealing of the cold rolled martensitic microstructure, which shows superior work hardening rate, low yield ratio, and high tensile strength. In this way, it is possible to enhance the mechanical properties of St12 steel toward those of DP300/600 steel. Compared with the conventional DP350/600 grade, a significant enhancement in the work-hardening behavior can be achieved with acceptable strength-ductility balance compared with the usual trend seen in steels. As a result, it was concluded that cold rolling of the initial martensitic microstructure before intercritical annealing is a viable approach for processing DP steels with enhanced mechanical properties for industrial applications.

    Keywords: Dual phase steels, Grain refinement, Chain-network martensite morphology, Mechanical properties, Strain hardening rate
  • H. Sharifi *, R. Mazahery, I. Ebrahimzade, A. Rajaee, B. Sadeghi Pages 18-26

    In this paper, an advanced thermo-mechanical treatment was conducted on AISI 316 austenitic stainless steel. At first, three samples were rolled at the ambient temperature, the temperature of -20 ºC (dry ice and ethanol) and -196 ºC (liquid nitrogen). Then, the samples were annealed at 800 ºC in the time range of 1 to 15 minutes. In each step, the microhardness values of the samples were measured. Microstructural investigations were conducted using optical and SEM microscopes and Clemex software. Results showed that microhardness of the samples increased due to the formation and precipitation of carbides. The formation of martensitic microstructure after the rolling process was revealed by X-ray diffraction analysis; and by decreasing the rolling temperature, the peak intensity increased. Also, by increasing the annealing time in each step, the volume fraction of the reverted austenite and the intensity of the austenite peaks increased.

    Keywords: Austenitic stainless steel, Ultrafine Grain, Thermo-Mechanical Processing, X-ray Diffraction Analysis, Mechanical properties
  • M. M. Karkehabadi *, A. Kermanpur, A. Najafizadeh, K. Kiani Pages 27-37

    In recent years, great attention has been paid to the development of high manganese austenitic TWIP steels exhibiting high tensile strength and exceptional total elongation. Due to low stacking fault energy (SFE), cross slip becomes more difficult in these steels and mechanical twinning is then the favored deformation mode besides dislocation gliding. Chemical composition along with processing parameters has profound effects on SFE and mechanical properties of TWIP steels. In this work, artificial neural network (ANN) models were developed to predict tensile properties of these steels. In these models, %Mn, %Al, %Si, %C, cold rolling reduction, strain rate, annealing temperature, and time were chosen as input, while engineering yield strength (Y.S.), tensile strength (T.S.) and total elongation (T.E.) were considered to be output parameters. The network models were trained for each output individually. A reasonable agreement was found between the results of tensile tests and the predictions, showing the robustness of the present ANN models. The developed models can be used as a guide to achieve high strength and ductility by (i) alloy design or (ii) controlling processing parameters through the strain-induced twinning process.

    Keywords: Artificial neural network, High manganese austenitic steel, TWIP steel, Mechanical properties, Stacking fault energy
  • H. Shafyei *, K. Amini Pages 38-44

    In this study, the effect of cryogenic treatment on microstructure, hardness and wear behavior of cold work tool steel containing 5 wt. % Cr used in the production of cold rollers was investigated. For this purpose, the starting and finishing temperatures of the martensite transformation were determined by dilatometer test (234 °C and -102 °C, respectively). Then, quenched-tempered heat treatment (QT) and quenched-cryogenic heat treatment for 24 hours-temper (DCT) were performed on the samples. In the next phase, microstructure of the samples was evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and their hardness and wear behavior were studied by pin-on- disk wearing machine. In order to determine the wear mechanisms, the level of the wear effect was investigated by SEM. The obtained results showed that in deep cryogenic treatment compared to the quenched-tempered treatment, due to the transformation of retained austenite to martensite, precipitation of fine carbides, increased volume fraction of carbides and more appropriate distribution of these carbides, led to an increase in hardness and wear resistance to 3HRC and 30%, respectively. The study of the worn- out surface and its products showed that the wear mechanism was adhesive wear along with tribochemical wear. Deep cryogenic treatment reduced the amount of adhesive wear.

    Keywords: Cryogenic heat treatment, Cold work tool steel, Retained austenite, Wear resistance
  • H. Rastegari *, M. Lakhi, M. Farazi, M. R. Haghighi Pages 45-53

    The aim of the present research is to simulate the heat treatment process of the autogenous grinding (AG) shell liner of Cr-Mo steel in different quenching media (still air, air blowing, oil and water) using ABAQUS software, in order to calculate the cooling curves. In this regard, the microstructure and hardness for each arbitrary point within the mill liner's bulk could be predicted based on using the CCT diagram of Cr-Mo steel and superimposing the cooling curves. Moreover, the results of the simulations and the industrial data were compared to prove the validity of the simulation. The findings showed that the microstructure of pearlite, duplex pearlite+bainite and martensite were achieved by quenching, using air fan, oil and water, respectively. The simulated cooling curves were consistent with the measured cooling curves, and simulation could be a promising method for designing the heat treatment process for mill liners.

    Keywords: Cr-Mo steel, Simulation, Heat treatment, Cooling curve, Microstructure
  • J. Rasti * Pages 54-61

    In this study, the hot deformation processing map of AISI 304 austenitic stainless steel in two initial grain sizes of 15 and 40 μm was investigated. For this purpose, cylindrical samples were used in the hot compression test at the temperature range of 950-1100 °C and the strain rate of 0.005-0.5% s-1. At first, the relationship between the peak stress and Zener-Hollomon parameter was obtained and their microstructures were studied, then the strain rate sensitivity and the processing map were determined at the strains of 0.5 and 0.7. It was found that in the aforementioned temperature range and strain rate ,the prevailing softening mechanism was the dynamic recrystallization process. Instability regions were observed at lower temperatures and higher strain rates, associated with the occurrence of necklace phenomenon in both grain sizes, as well as the semi-necklace structure in coarse-grained steel and in fine grain steel to the formation of geometrically necessary dislocation along the grain boundaries. Moreover, it was associated with the development of cell structure interior the grains. In addition, with an increase in temperature and a decrease in strain rate, both the power dissipation efficiency and the strain rate sensitivity increased, indicating an increase in the volume of the recrystallized materials in a constant strain and a decrease in the localization of the plastic flow.

    Keywords: Austenitic stainless steel, Dynamic recrystallization, Processing map, Grain size effect