Materials Science Forum Vols. 783-786

Abstract: The aim of this research work is to study the inter-relationship, under controlled industrial conditions, among skin pass reductions, surface topography (characterized by 2D and 3D) roughness parameters, stampability and painted surface finish quality for automotive steel sheet stampings.Different surface textures obtained from cold rolling finishing have been evaluated in terms of paintability tests (rating and spectral curve) and tentatively related to roughness parameters (2D and 3D) obtained from the cold finished sheets. Some relevant tendencies have been established amongst these parameters.The results presented here are in accordance with other recently published research showing that there is a clear relationship between these parameters, and that further detailed studies are needed.

Abstract: A 400 MPa yield strength structural steel plate with enhanced weldability was produced by using advanced steel making technology and thermo-mechanical controlled processing technique. A microstructure consisting of acicular ferrite (3~8 μm) and polygonal ferrite was observed in the rolled plate, which exhibits a yield strength ≥ 420 MPa, tensile strength ≥ 560 MPa, elongation ≥ 26 % and charpy impact toughness ≥ 300 J at-40 °C. Three-wire flux copper backing submerged arc welding with heat input of 230 kJ/cm was applied to butt weld the 36 mm thick plate, and defect-free joint with satisfactory mechanical properties were produced. The coarse grain heat affected zone (CGHAZ) contains mostly intra-granular nucleated ferrite plus a few grain boundary ferrite and ferrite side plate, and shows charpy impact toughness ≥ 90 J at-40 °C. The enhancement impact toughness of CGHAZ resultant from high heat input welding is due to improvement of intra-granular ferrite formation induced by Ca and Ti containing oxides and sulphides.

Abstract: Effect of Mn, Si and Cr on spheroidization of cementite in Fe-1mass%C steel has been investigated over a range of austenitizing temperatures. In Fe-1C steel, a fully spheroidized structure is obtained but some large cementite particles are formed. The addition of 1.5 mass% Si or Cr accelerates spheroidization of cementite. An addition of Cr remarkably refine the cementite particle size, but the influence of Si addition on the cementite particle size is not remarkable. A fully spheroidized structure fails to develop in steel with the addition of 1.5% Mn under the condition used in present study. Some lamellar cementite still exist in the 1.5Mn steel. The pearlite-promoting effect of Mn is possibly attributed to the inhomogeneous distribution of cementite particles during the intercritical austenitization.

Abstract: The obtainment of ultrafine grain microstructures, by the application of process parameters which are potentially feasible under industrial conditions, is attractive to develop a new generation of low alloy steel (Ultrafine Grain Steel, UFG) characterized by high strength and toughness, good cold/warm formability, environmentally-friendly process. The ferrite grain size refinement beyond existing levels by means of hot rolling mills, without requiring drastic plant changes, can be achieved by lowering the rolling temperature down to the range A_e3 - A_r3 in the finishing stands. In this temperature range different metallurgical mechanisms may take place. Austenite recrystallization is slower and there is a greater chance of obtaining non-recrystallized deformed austenite (pancake), which after phase transformation will give finer ferrite (Heavy Gamma Deformation). Or, in alternative, Deformation Induced Ferrite Transformation can occur especially in C-Mn steels, promoting the formation of ultrafine ferrite grains (DIFT). Most of the existing studies on UFG steel focus on flat products. In this paper the mechanisms to be exploited for producing UFG long products are identified and examined on different low and medium carbon non-alloyed steels, as the common grades used for fastener applications. In particular, Heavy Gamma Deformation and DIFT are investigated through laboratory tests aimed at determining the process parameters affecting the two mechanisms in different ranges of chemical composition. On the basis of the results found, some basic concepts for industrialization on modern hot rolling mills will be given.

Abstract: The potential to utilize controlled thermal processing to minimize banding in a DP780 steel with 2 wt pct Mn was evaluated on samples processed on a Gleeble® 3500 thermomechanical processing simulator. All processing histories were selected to result in final dual-phase steel microstructures simulating microstructures achievable during annealing of initially cold rolled sheet. Strip samples were processed to evaluate the effects of heating rate, annealing time, annealing temperature, and cooling rate. The degree of banding in the final microstructures was evaluated with standard light optical microscopic techniques. Results are presented to illustrate that the extent of banding depended on control of both heating and cooling rates, and a specific processing history based on a two-stage heating rate can be used to minimize visible banding in selected final heat treated products.

Abstract: Recrystallization-precipitation-time-temperature (RPTT) diagrams were experimentally determined for two microalloyed steels with V and Nb, respectively, at a strain of 0.35 and a strain rate of 3.63 s^-1. From the RPTT diagrams, and applying the classic theory of nucleation, the nucleation rate was calculated for both steels. In order to determine the mentioned magnitudes, several parameters were calculated, such as: the Zeldovich factor (Z), the energy of formation of the nucleus (ΔG), the driving force for precipitation (ΔGv), the critical radius for nucleation (Rc), and the dislocation density at the start of precipitation (ρ), among others. The calculated data has made it possible to clarify the shape of precipitation start and finish curves and to plot the nucleation rate as a function of temperature. The number of precipitates was calculated by integration of the nucleation rate expression. In this way, substantial differences were established between the two types of microalloyed steels, including the final size of the V(C, N) and Nb (C, N) precipitates.

Abstract: Specific types of non-metallic inclusions are known to act as heterogeneous nuclei for the formation of acicular ferrite, which provides excellent toughness. By increasing the amount of acicular ferrite in the microstructure, the properties of HSLA steels can be optimized significantly.Although the formation of acicular ferrite caused by heat treatments (thermomechanical treatments or welding) is quite well described in literature, there is less information to find about the formation of acicular ferrite immediately out of the liquid melt. Within the present study experiments on laboratory scale are carried out simulating the influence of cooling conditions and Ti-content on size, chemical composition and morphology of non-metallic inclusions and consequently on the amount of acicular ferrite. All experiments were carried out with a dipping test simulator enabling very well controllable cooling conditions. Optical microscopy in combination with special etching methods as well as SEM/EDS-analysis was used for microstructure and inclusion characterization.

Abstract: Development of creep-resistant 8mol% Yttria-stabilized Zirconia (YSZ) ceramic has received much interest due to its potential use in fuel cells and thermal barrier coatings. In this research, Spark Plasma Sintering was used to develop a high-density 10 mol% La₂0₃ + YSZ composite. Compressive creep testing was performed at 1300 oC at 45 – 78 MPa load. The mechanism of plastic deformation of the composite was studied using Scanning Electron Microscopy and X-Ray Diffraction. The results suggest that lattice diffusion and grain boundary sliding were the active creep mechanisms.

Abstract: Cubic Yttria-stabilized zirconia (c-YSZ) is a candidate material for high temperature applications due to its chemical inertness, high thermal stability and low thermal conductivity. In addition, due to c-YSZ’s high radiation stability, there is a new interest in this ceramic for thermal insulating cladding material in Supercrticial Water Nuclear Reactors (SCWR). However, due to the aggressive environment of supercritical water, the degradation behaviour of the ceramic must be well understood prior to its industrial application. In this research, moderately dense 8 mol% Yttria stabilized zirconia (YSZ) as well as YSZ composites containing 5, 10 and 15 mol% CeO₂ were fabricated via Spark Plasma Sintering (SPS) and subjected to static corrosion testing in supercritical water (SCW) at 400°C and 31MPa. Weight loss and crystal lattice changes associated with SCW exposure were studied.

Abstract: There is a need to enhance or develop high temperature capabilities of structural materials for advanced coal‐fired power plants. These materials require a combination of high temperature strength, creep resistance and corrosion resistance in the oxygen‐rich and hydrogen‐rich high pressure environments. In this study, atomized Ni‐20Cr (wt.%) powder was mechanically milled with Y₂O₃ nanopowder (30‐40 nm powder size) to produce an alloy with a chemical composition of Ni‐20Cr‐1.2Y₂O₃ (wt.%) alloy using high energy ball milling. To minimize agglomeration during milling, 1 wt.% stearic acid was added to the powder mixture prior to milling. Microstructural characteristics of the powder were primarily characterized by the X‐ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The crystallite size and lattice strain were measured by XRD whereas powder morphology (powder size, shape) was studied by SEM. A milling time of 2 h was found to be optimal for the purpose that yttria particles are not dissolved yet uniformly distributed. Subsequently, the milled powder was consolidated into bulk specimens (12.5 mm in diameter) via spark plasma sintering (SPS) at 1100 °C for 30 minutes. Following SPS, the density and hardness of the specimens were measured. Microstructural characterization of the SPSed specimens was performed using SEM and TEM. The microstructural characteristics were correlated with the measured mechanical properties.