Papers by Keyword: Controlled Cooling

Abstract: A microstructure consisted of granular bainite, upper bainite, acicular ferrite, polygonal ferrite, and a little of pearlite was obtained by controlling rolling and cooling in marine steel. The grain size of the two steels was refined to 5-9 μm. The size of the MA island was about 3-6 μm. Both steels had higher tensile strength, yield strength, elongation and hardness, and the performance of steel B was better than that of steel A. Both steels exhibited large and deep dimples at -80 °C impact fracture. The longitudinal impact energy of the specimens was above 250J, and the steel B even exceeded 300J. Its longitudinal impact performance was better than lateral impact performance. Both steels had achieved good performance due to fine grain strengthening, MA island strengthening, precipitation strengthening and dislocation strengthening caused by controlled rolling and cooling. The alloying element nickel effectively improved the low temperature impact toughness of the experimental steel. Excellent impact energy of both steels was attributed to higher texture strength for RD specimens.

Abstract: Simulation of the rolling and controlled cooling sequence for bainitic steel rods was the general objective of the paper. The main focus was put on exploring possibility of prediction of the retained austenite occurrence in TRIP assisted bainitic steels. Existing discrete phase transformation models require long computing times and their application to optimization of industrial processes is limited. Therefore, a model based on the modified JMAK equation was proposed. The occurrence of the retained austenite was predicted by carbon distribution calculations in the austenite during bainite transformation. This model was implemented into the FE software for simulation of cooling of rods. The model was verified by comparison of results with the physical simulations during rolling in the pilot mill and during cooling. The first part of the paper contains thermal-mechanical-microstructural simulations of rod hot rolling process. The objective of this part was to determine temperature and grain size distribution at the rod cross section at the beginning of phase transformations. FE simulations of the cooling were performed next. Correlation between cooling parameters and the volume fraction of the retained austenite in rod was determined.

Abstract: The mechanical properties and microstructures of three different VN, Nb, V-Nb microalloyed rebars were investigated by using tensile testing machine, impact machine, metallographic microscopy, scaning electron microcopy, transmission electron microscopy and X-ray diffraction apparatus. The results showed that the microstructure of V-Nb microalloyed specimen is consisted of ferrite, pearlite and a small amount of fine bainite (6.7wt%), and obvious effect of grain refinement was obtained with more than 10 size grade of ferrite grain, showing optimal comprehensive properties. SEM micrograph of tensile fracture surface for V-Nb microalloyed 500MPa high-strength rebar is dimple and ductile, ductile-brittle transformation temperature is lower than-30°C, which has good plasticity-toughness and impact toughness at low temperature. The results of precipitates have shown that a large number of small and dispersive V(CN) and Nb (CN) precipitates with size of 5~30nm are formed, good effect of precipitation strengthening was achieved in 500MPa high-strength rebars produced by different microalloying and controlled cooling technology.

Abstract: In this paper, critical temperature of phase transformation and continuous cooling transformation (CCT) curve of S87B were determined by hot uniaxial compression tests. At several of cooling speed, the microstructures were studied. The results indicate that the critical temperature of phase transformation become lower with cooling speed increasing, the min interlamellar spacing of pearlite was 0.125μm when the cooling speed was 4°C/s, the best cooling speed of phase transformation area was during (3-4)°C/s..

Abstract: Controlled cooling of H-beam after rolling, can change the microstructure consituent,improve the strength and improve the general mechanical property and service performance. According to actual product, the rational thermal boundary condition adopted, three dimensional FEM model is established. Spray cooling is used. Transient temperature field and stress field is simulated by the FEM software ANSYS/Multiphysics when H-beam is cooled. The four kinds of cooling scheme are designed. Through analysis of the relation of temperature field with stress field, the main reason of producing residual thermal stress is the section temperature difference in the cooling process of H-beam after rolling.

Abstract: The paper presents the methodology and the results of determination of emissivity coefficient for nickel alloy Inconel 718 in the temperature range predicted for thermomechanical processing of these alloys and its application for correction of temperature during cooling and modeling. The idea presented in the paper is an iterative use of automatic system of temperature measurement and control, facilitated with thermocouples and pyrometer gauge to provide temperature dependent characteristics of the alloy. In addition to estimation of temperature dependence of the emissivity coefficient, the results of cooling tests for simple and complex geometry of nickel alloy forgings are contained, obtained with modeling based on assumption of the determined characteristics. To verify obtained characteristic, they were used in modeling of cooling and compared to those obtained in measurements based on assumption of constant value of emissivity coefficient. Measurement errors obtained were used to indicate show the significance of application of functional dependence in comparison with constant value, as well as, magnitude of error made during temperature measurement or modeling, when neglecting the temperature dependence.

Abstract: The finite element analysis software soft ANSYS is used for researching H-beam hot rolling finite element model, temperature field and stress strain field. Then find the changes law in different water cooling conditions of the temperature field temperature distribution and the stress strain field. Find the heat coefficient of H-beam by using the optimization analysis to reduce the differences of the temperature and the stress strain value, and also reduce the deformation in H-beam cooling process. The controlled cooling parameters of H-beam under optimal conditions are determined by the simulation experiment and comparative analysis. While exploring the changes of H-beam microstructure, stress and hardness can provide the reference for the making and designing of reasonable controlled cooling technology.

Abstract: In the present paper, controlled cooling of Fe-C-Mn-Si multiphase steel was conducted by a laboratory hot rolling mill. The results show that polygonal ferrite, granular bainite and the stable retained austenite can be obtained through hot deformation and subsequent two steps cooling pattern. The amount of ferrite increased with the duration of intermediate air cooling during controlled cooling. The formation of the bainitic ferrite resulted in the carbon concentration enrichment in austenite further during the simulated coiling. This increases the stability of the remaining austenite. Satisfactory mechanical properties can be obtained through hot rolling process and two steps cooling pattern in this work due to the TRIP effect of the stable retained austenite.

Abstract: In the present paper, controlled rolling and controlled cooling of Fe-C-Mn-Si multiphase steel was conducted by a laboratory hot rolling mill. The results show that ferrite (grey), granular bainite (black) and retained austenite (white) and/or MA islands (white) are observed in a color etched LOM micrograph. The presence of the retained austenite is confirmed by SEM observation. Controlled rolling and controlled cooling technology contributes to the improvement of the microstructure. Excellent mechanical properties for Fe-C-Mn-Si multiphase steel are attributed to the TRIP effect of the stable retained austenite.

Abstract: In order to improve the comprehensive mechanical properties of the steel, the heat treatment software COSMAP is used to simulate the rolling and controlled cooling of I-beam. The numerical simulation shows that: when the cooling rate is controlled at 10 °C/s around, the mechanical properties of controlled cooling can be obviously improved. The strength and hardness can be improved on the condition of ductility and toughness ensured, while the amount of residual austenite can be reduced significantly, which provide a theoretical basis for further optimization of the heat treatment process.