Advanced Materials Research Vols. 15-17

Abstract: Controlled cooling on the runout table is a crucial component in the production of highly tailored steels since it has a strong influence on the final mechanical properties. High efficiency heat transfer in impinging jet cooling makes this an important method for heat transfer enhancement. The purpose of this study is to develop an experimental database for modelling of boiling heat transfer for bottom jet impingement that occurs during runout table cooling in a steel mill. Experiments have been carried out on a pilot scale runout table using stationary plates, with focus on the effect of water flow rate and nozzle inclination to the overall heat transfer rates. Volumetric flow rates and inclination angles are in the range of 35-55 l/min and 0-30º, respectively. Temperatures on the test plates are measured internally very close to the surface during cooling for the purpose of reducing thermal lag and receiving better data responsiveness. These measurements are taken at the impingement point and several streamwise distances from the impingement point. From the above measurements transient cooling data on the hot steel plate by bottom jet impingement has been analysed.

Abstract: Low alloy multiphase TRansformation Induced Plasticity (TRIP) steels offer an excellent combination of a large uniform elongation and a high strength. This results from the composite behaviour of the different phases that are present in these steels: polygonal ferrite, bainitic ferrite and a martensite/austenite constituent. In order to obtain a clear understanding of the behaviour of the different constituents within the multiphase steel, they were prepared separately. The stress-strain relationship of the different single phase and multiphase steels were simulated with physically based micromechanical models. The model used to describe the stress-strain curves of the separate phases is based on the Mecking-Kocks and Seeger-Kocks theories and uses physical properties such as the microstructural properties and the chemical composition of the different phases. Strain-induced transformation kinetics, based on a generalized form of the Olson-Cohen law, were used to include the influence of the transformation of the metastable austenite. Static stress-strain properties of multiphase steels were modelled by the successive application of a Gladman type mixture law for two-phase steels. The model yields detailed information of stress and strain partitioning between the different phases during a static tensile test. A model for the dynamic stress-strain properties of ferritic steels is also proposed.

Abstract: Recently, there has been a large interest in the development of low carbon steels with ultra fine grain structure using lean chemistries. Although these steels typically have superior strength, the lack of work hardening capability limits the uniform elongation and thus the formability of these kinds of steels. It has been reported by Tsuji and co-workers (2002) that straining of martensite as an initial structure can yield an ultra fine grain structure with good combination of strength and ductility. However, the detailed mechanism of the grain refinement has not yet been clarified. In the present work, the annealing behavior of a low carbon martensitic structure with and without deformation at room temperature has been systematically studied. It is proposed that the process of concurrent softening due to recovery and recrystallization and precipitation of carbides is different for the deformed and undeformed materials. Further, preliminary results have been found on the role of substitutional alloying elements such as Mo or Cr on the kinetics of the softening processes.

Abstract: Lotus-type porous Fe-25wt.%Cr and Fe-23wt.%Cr-2wt.%Mo alloys were fabricated by continuous zone melting technique in pressurized hydrogen gas. After applying a high temperature nitriding treatment, the fabricated Lotus-type porous nickel-free stainless steel absorbed larger amount of nitrogen compared with non-porous alloy of the same composition since the surface area exposed to the gas is larger in the porous samples. In the Lotus-type porous Fe-25wt.%Cr and Fe-23wt.%Cr-2wt.%Mo alloys the nitrogen concentration after the nitriding achieved was approximately 1.2 wt.%. Only austenite peaks were detected in the profile of both Fe-Cr-N alloys after the nitriding treatment. Neither CrN nor Cr2N were identified by XRD in any specimen after the nitriding.

Abstract: Surface graphitization is a well known defect that occurs when low carbon steel strip is batch annealed. A small addition of chromium (about 0.04 wt %) can be used to minimize the surface graphitization. However chromium and some other alloy elements, such as manganese used in this class of steel, have higher affinities for oxygen than iron. Therefore it is possible for them to be oxidized during batch annealing in a reducing environment to iron. Selective oxidation of these two elements gives rise to a risk of forming different surface defects that may affect the quality of the tinned surface. The edge defect is characterized as a region of low reflectivity on the tin plate product caused by grain boundary precipitates. A porous tin coating with a dull appearance is produced in the affected areas. Not only are the aesthetical values of the finished tin plate product affected but the corrosion resistance is also reduced. In this investigation surface oxides formed at grain boundary of low carbon steels annealed at 700°C in 5% hydrogen 95% nitrogen atmosphere were characterized using scanning electron microscopy and transmission electron microscopy. Two different oxide particles (Fe,Mn)O and MnCr2O4 were observed at the grain boundaries with the former five-fold coarser than the latter. It was found at the annealing temperature of 700°C, that the mean particle size of the (Fe,Mn)O depends on the manganese content, while the mean particle size, and distribution of the MnCr2O4 was dependent on chromium, but independent of manganese. The coarse (Fe,Mn)O precipitates pose no potential risks to electrolytic tinning as they will be removed by the pickling operation prior to tinning. Controlling the chromium content of the steel can minimize the potential risks posed by the MnCr2O4 to the quality of electrolytic tinning.

Abstract: In this research, a new type of surface carburizing method which combines superplastic phenomenon and carburizing process called superplastic carburizing (SPC) was introduced. Thermo-mechanically treated duplex stainless steel (DSS) with fine grain microstructure and exhibits superplasticity was used as the superplastic material. The SPC was conducted at temperatures ranging from 1123 K to 1223 K for various durations. Initial loads of 25 MPa, 49 MPa and 74 MPa were applied to give the superplastic deformation effect on the carburized specimens. Metallographic studies revealed a thick, uniform, smooth and dense morphology of hard carbon layer formed at the surface of fine grain DSS. The resulting case depth of carbon layers were between 15 μm to 76 μm. A remarkable increase in surface hardness was observed in the range of 600 HV to 1600 HV. The kinetics of this process in terms of carbon diffusion and its variation with processing time and temperature was achieved using Arrhenius equation. Activation energy (Q) was determined as 151.87 kJ/mol. Based on the results obtained, SPC process can significantly enhance the surface properties of DSS.

Abstract: The deformation behaviour of martensite and its effect on tensile properties of a lowcarbon dual-phase (DP) steel were investigated. DP steel samples with different martensite contents and morphologies were produced after intercritical annealing at different temperatures using low and high heating rates. Two distinct martensite morphologies were obtained for low and high heating rates. The investigated steel showed the unusual results that the true fracture stress and strain were found to increase with the martensite volume fraction. The plastic deformation of martensite was considered to be responsible for these results. Experimentally, it was observed that the martensite in DP steels with greater than 25-30% martensite can deform plastically during tensile straining. Finally, the effect of tempering on the martensite plasticity was also evaluated. It was found that the tempering process and an increase in the martensite content have a similar effect on promoting martensite plasticity.

Abstract: Effects of acicular ferrite and retained austenite on the mechanical properties of bainite-base steels were investigated. Various morphology and volume fraction of constituent phases have been obtained by control of hot rolling conditions and alloy compositions. It has been shown that the steels containing retained austenite have better combinations of strength and ductility than the ones with no retained austenite. However, there is no noticeable change in DBTT by the incorporation of retained austenite in the microstructure since retained austenite exists as fine particles. On the other hand, DBTT of the steels are largely affected by the presence of acicular ferrite in the microstructure. EBSD analyses of fractured Charpy specimens show that cracks are deflected within the morphological packet of acicular ferrite, indicating its role in reducing the effective grain size of the steels.

Abstract: Effects of deformation at austenite region and cooling rate on the microstructure and mechanical properties of low carbon (0.06 wt. % C) high strength low alloy steels have been investigated. Average grain size decreased and polygonal ferrite transformation promoted with increasing deformation amount at austenite region due to increase of ferrite nucleation site. Microstructure was also influenced by cooling rate resulting in the development of a mixture of fine polygonal ferrite and acicular ferrite at 10°C/s cooling rate. Discontinuous yielding occurred in highly deformed specimen due to the formation of polygonal ferrite. However, small grain size of highly deformed specimen caused lower ductile-to-brittle transition temperature than slightly deformed specimen.

Abstract: including, external oxide layers, internal grain boundary oxidation structures as well as many other forms of internal oxidation. During the present study, needle like grains of hematite have been observed within the top layers of a number of external oxide scales formed during simulated reheat of 316L stainless steel. It is believed that these needles are caused by the decomposition of an iron rich spinel (approximated to magnetite) along a preferred crystal direction within the spinel grains. The needles have been studied using optical microscopy, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD).