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Iron and steel society of Iran - Volume:13 Issue: 2, Summer and Autumn 2016

International Journal of iron and steel society of Iran
Volume:13 Issue: 2, Summer and Autumn 2016

  • تاریخ انتشار: 1395/11/11
  • تعداد عناوین: 7
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  • A. Mostafapour, A. Ebrahimpour *, T. Saied Pages 1-6
    Transformation induced plasticity (TRIP) steels have a vast application in automotive industry because of their high strength, high ductility and hence excellent energy absorption capacity. These characteristics of TRIP steels are due to the existence of retained austenite in their microstructures in the ambient temperature, which transforms to the martensite phase during deformation. The microstructure of TRIP steel contains various phases and in the past-published studies mainly the volume fraction of retained austenite was investigated and there is not a quantitative comprehensive investigation about all phases in the microstructure of this steel. The main goal in this study is a comprehensive qualitative and quantitative investigation in various phases of TRIP steel microstructure. Therefore, a TRIP steel with chemical composition of 0.2C 1.43Si 1.5Mn was produced and its complicated microstructure which contained ferrite, bainite, martensite and retained austenite was investigated using X-Ray Diffractometry (XRD), optical microscopy (OM), field emission scanning microscopy (FE-SEM) and Electron backscatter diffraction (EBSD). The OM and FE-SEM results were used only to qualitative studies and identification of the morphologies of the phases but the EBSD results and functions were used to qualitative and quantitative studies. The volume fractions of retained austenite, ferrite涨꞊ and martensite phases were calculated and the amounts of 11%, 82% and 7% were obtained, respectively. The volume fraction of retained austenite was also measured with XRD and the amount of 14.3% was obtained.
    Keywords: TRIP steel, XRD, FE, SEM, EBSD, Retained austenite
  • M. Alizadeh *, Y. Palizdar Pages 7-12
    Grate bar is an industrial part which is used in the indurating furnace of an iron ore pelletizing plant. Steel part of the grate bars are expected to have high resistance to atmospheric oxidation because of its contact with hot gases (up to 920 °C) during the normal operation of the process. In this study, different samples from several sections of used grate bars with apparent defects were selected. The microstructure of the selected samples from the affected areas was studied via optical and scanning electron microscopy with EDS analysis. The results showed that the internal oxidation would occur through the oxygen diffusion into the chromium carbide nets and caused the change of chromium carbide to chromium oxide. Furthermore, internal oxidation led to the separation of the grains from the steel matrix and resulted in the dusting phenomenon. In order to modify the steel microstructure, Ti element was added to the melt in different levels during the casting process and the results showed that the presence of Ti could modify the carbides structure and consequently improve the oxidation resistance of the steel. The grate bars with new composition were placed in the furnace and the monitoring of the consumption of the grate bar in Mobarakeh Steel Company pelletizing plant. In a six -month period revealed that the grate bar consumption decreased about 30%.
    Keywords: Heat resistant steel, Chromium carbide precipitation, Grate bar, Microstructure
  • M. Rakhshkhorshid, H. Rastegari Pages 13-18
    The main contribution of the present work is to investigate the capability of exponential type constitutive equation to model the warm deformation flow curves of a eutectoid steel in the temperaturerange of 620-770 °C and at the strain rates in the range of 0.01-10 s-1 conducted on a Gleeble-1500 thermomechanical simulator. Warm deformation in this temperature range facilitates the occurrence of dynamic spheroidization of cementite lamellae as a softening process as well as some instabilities and microstructural defects. The prediction capability of the examined model was assessed using the average absolute relative error (AARE) criterion. The obtained AARE with the value of 7.39% for warm deformation modeling of the tested steel showed the acceptable performance of the examined model.
    Keywords: Warm deformation, Flow stress, Exponential, type constitutive equation, Eutectoid steel, Dynamic spheroidization
  • M. Rafiei, M. Shamanian, M. Salehi, H. Mostaan Pages 19-25
    In this research, B4C-Ni cermet coating was sprayed on the surface of 4130 steel from B C and Ni feed-stock powders using high velocity oxy-fuel (HVOF) method. In order to characterize the tribological behavior of the coating, ball on disk wear tests were done at the ambient temperature and under the loads of 1, 3 and 5 N. Phase analysis of the coating after spraying was studied by X-ray diffractometery (XRD). Microstructure of the coating and wear track was evaluated by scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS). The microhardness test was done to measure the hardness of the produced coating. It was found that a good coating with suitable interface and no significant pores and cracks in the microstructure of coatings was formed. The main wear mechanism of the coating was delamination with some oxide layers due to the frictional heat during wear test.
    Keywords: B4C-Ni coating, Microstructure, Wear test, Delamination
  • M. Imaninezhad *, T. Yan, A. Najafizadeh Pages 26-30
    Austenite is a semi-stable phase in most stainless steels that deforms to martensite under Md30 and forms martensite type ά and ε due to the deformation in the steels. Since the distribution of strain induced martensite plays an important role in achieving desired properties, the main objective of the present work is to model martensite distribution of ά during cold rolling using finite element method and Olsen-Cohen model. In this study, the strain induced martensite transformation of 301 stainless steel during cold rolling has been simulated by ANSYS software. First, the mesh sensitivity analysis was performed and mesh optimization was set to stimulate the strain induced martensite transformation of 301 stainless steel during cold rolling. Martensite fractions in cold-rolling was simulated and compared with experimental data. Finite element analysis was performed to obtain strain and stress during cold rolling. The amount and distribution of martensite during cold rolling has been modeled. The highest stress level was observed and applied on a friction plate which was in contact with rollers and, as a result, was under the most friction; thus, the stress reduced away from the surface toward the center of the sheet. Moreover, a similar phenomenon was observed for changes in the strain. These results were also compared with experimental data that had been obtained with X-ray diffraction, with the use of a Ferritoscope, X-ray diffraction and experimental results.
    Keywords: Cold rolling, Austenitic stainless steel, Martensite transformation, Finite element, Stress-strain
  • M. Tavoosi *, M. Nourbakhsh, S.R. Hosseini Pages 31-39
    The development of nano/ultrafine bainitic structure in AISI 52100 steel was the goal of this study. For this purpose, the AISI 52100 the specimens were austenitized at 1050 °C for 60 min followed by low temperature austempering treatment at different temperatures and times. The austempered samples were characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and tension test. According to achieved results, the microstructure of AISI 52100 steel after the austempering treatment consisted of ultrafine bainitic ferrite plates and retained austenite with two morphologies of micrometer-block and fine film. At the austempering temperature up to 250 °C, the micrometer-blocky morphology of austenite completely vanished from the microstructure, and the strength and ductility increased to about 2000 MPa and 7 %, respectively. By increasing the austempering temperature to 300 °C, the strength and ductility reduced (to about 1808 MPa and 3 %) simultaneously as a result of changing in the shape and size of bainite phase.
    Keywords: Nanostructures, 52100 steel, X-ray diffraction, Mechanical properties
  • B. Mansouri *, H. Montazer Hojat Pages 40-45
    Structural steel sections are mainly used in beams and columns of building frames. Iran National Steel Industrial Group is among the oldest and largest producers of beams in Iran. It has two beam production lines, namely Line 630 and Line 650. In this study, the mechanical properties of manufactured beams in these production lines were compared. Based on the t-test results, the elongation is significantly higher in Line 650 products while tensile strength is higher in Line 630. However, the two lines do not show any significant differences in the yield stress. Mann-Whitney test was also used to determine if one of the mechanical properties significantly differs in the two lines products. Based on the test, the tensile strength, elongation and yield stress were significantly different in the two production lines. According to the Levene and Fisher test, Line 650 products were more homogeneous in terms of the tensile strength and yield stress while Line 630 products were more homogeneous with regard to elongation. Moreover, the length of the production line and the cooling time seem to affect the mechanical properties. In addition, an inverse relationship between tensile strength and beam elongation was observed in this study.
    Keywords: Beam_Mechanical properties_Student's t tests_Mann-Whitney test_Levene test