Copper and sulfur are the typical residual elements and impurities in steel. Previously, we reported the precipitation of very tiny particles of Cu2S in copper and sulfur containing steel by strip casting process. In the present paper, the morphologies of copper sulfides in strip casting low carbon steels were distinguishably investigated by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Four kinds of copper sulfide with different morphology were observed, namely duplex inclusion of oxide and sulfide (OS), plate-like copper sulfide (PS), shell-like copper sulfide surrounding the inclusions (SS), and nano-scale copper sulfide (NS), and their formation mechanisms were discussed. The OS is considered to firstly form as molten manganese silicate in molten steel, and grow up with the formation of sulfide inside of the silicate after the solidification of steel. The PS is considered to precipitate from the γ-Fe phase with plate like shape due to semi coherency with the γ-Fe matrix. The SS is considered to precipitate in lower temperature ranges on the other pre-formed inclusions such as MnS, oxide and also Cu2-xS. The NS is considered to form in the low temperature range of γ-Fe and especially in α-Fe phase as very tiny particles due to the high supersaturation, low diffusivity of component elements and the coherency with the α-Fe matrix. Based on this classification, formation stage of oxide, MnS and Cu2S was clarified and described as like TTT diagram.
Liquid Ni-C and Ni-C-Mg alloys (12.5 at% C) were solidified ultra-rapidly using twin rollers at various cooling speeds. A number of spheroidal graphite (S.G.) with a diameter of a few micron meters were observed by TEM in both of the alloys. At each center of S.G. in Ni-C alloy, a small different phase was recognized, in which some of the calcium, silicon, aluminum, potassium, magnesium and oxygen were detected. The result shows that S.G. in Ni-C alloy could be formed through heterogeneous nucleation on a nucleus formed from the impurities of graphite reagent. In contrast, S.G. in Ni-C-Mg alloy did not contain such kind of nucleus, having shown more roundish shape than that in Ni-C alloy. Supersaturated carbon in nickel has a tendency to crystallize at the surface of bulk nickel, which was recognized through carbon films precipitated on surface of all the samples. Accordingly, the S.G. in Ni-C-Mg alloy could be formed through nucleation at the interface between magnesium bubble and nickel, and growing up to the center. X-ray diffraction of both alloys proved two types of nickel containing 1.2-1.3% C and 7.5-8.5% C, respectively. In case of Ni-C alloy solidified at a rather slow cooling speed, Ni3C in addition to the two nickel phases was confirmed. For the samples solidified at a most rapid cooling speed, only one type of nickel containing 1% C was shown. The electron diffraction of carbon film on nickel surface showed that basal plane of graphite preferentially covered the nickel.
Influence of the surface characteristics of metallic substrate on the formation of initial solidification structure of steel was experimentally studied using AISI 304 stainless steel and the surface treated copper plates. The molten steel was dropped onto the copper substrate with or without surface undulation at the room temperature or heated to 673K, respectively. The other substrates had dimples machined by shot blasting or Rockwell hardness tester. The solidification structure and the dendrite arm spacing were analyzed on each solidified shell, and heat flux across the molten steel and the substrate was evaluated with the measured substrate temperature. It was found that the roughness of the substrate surface in a range of 10-16 μm had little influence on the heat transfer. The dendrite convergent spot (DCS) was evaluated by the direction of the primary dendrite arms near the surface. DCSs were appeared at 0.2-0.5 mm intervals on the flat substrate, and the interval of DSCs was broadened as increasing the substrate temperature. DCSs were appeared on the ridge of the dimples of the substrate, and another DCS was appeared between the pair of dimples when the distance of ridges exceeded 0.5 mm. Mechanism of DCS formation on the flat substrate was discussed. Furthermore, the influence of thermal diffusion during initial rapid solidification on the intervals of DCSs was evaluated.
In order to reduce the rolling force and the roll wear, the lubricants have been used in hot sheet rolling of steel, but the lubrication behavior at the interface between roll and workpiece in hot steel rolling have never been well understood. On the other hand, hot rolling process with high reduction in thickness have been developed to produce the ultra-fine grained steels. The high reduction in hot rolling causes some troubles such as the increase of the rolling force, the occurrence of friction pick up and so on. To solve these problems, the lubrication behavior at the interface between roll and workpiece in hot steel rolling must be quantitatively understood. In this paper, the coefficients of friction were measured by using the newly developed simulation testing machine for hot rolling. The effects of lubricant factors such as the compositions and the viscosity of the base oil and the additive agents on the coefficient of friction were investigated. From these experimental results, the lubrication behavior at the interface between roll and workpiece was investigated and the lubrication mechanism was proposed.
The experiments for cold rolling of stainless steel sheets were made in order to clarify the effect of methods of roll polishing and surface roughness of rolls on the surface characteristics of rolled sheets such as surface gloss, ratio of flat area and so on. Then, the surfaces of rolled sheets were observed with a microscope and a scanning electron microscope. As a result, it has been found that the surface of sheets rolled with the rolls polished in the axial direction with #800 polishing paper, which shows the largest gloss in the present experiments, is considerably flat in spite of having thin uneven linear defects in the normal direction to the rolling one.
A novel straining process characterized by multidirectional deformation was proposed to fabricate large scale, ultrafine-grained (UFG) steel for a thick plate. To impose a large strain into the whole area of material, free forging technology was applied. The principles of grain refinement in basic experiments using small-scale samples were directly applied to test production of large-scale samples. Three-dimensional finite element simulation (FE simulation) was utilized in developing the new process from the viewpoints of the prediction of press load, microstructure, and plate shape. The proposed UFG process was tried using a practical manufacturing press having maximum press load of 3500 ton. The microstructure and hardness of the plate were made as predicted. As a result, it is possible to fabricate UFG steel plates with 25 mm or more thickness by free forging at warm working temperatures.
Electrodeposition of Zn with V was tried in un-agitated or agitated sulfate solutions containing Zn2+ and VO2+ at pH 0-3 and 40°C under galvanostatic conditions. XPS spectra of deposits showed that V was involved in deposited Zn in form of V oxide by hydrolysis of V ions. The content of V in deposits increased with increasing pH in solution and current density. These conditions appear to accelerate the hydrolysis of V ions as a result of an increase in hydrogen evolution in cathode layer. SEM and EPMA images of deposits revealed that V in deposits segregated at edge of layered platelet crystals of Zn. The agitation of electrolyte decreased the content of V in deposits, but decreased the segregation of V oxide. Anodic polarization curves for Zn dissolution in 3% NaCl solution were polarized by codeposition of V oxide with Zn in the range of V content less than 5 mass%. The corrosion current densities of deposits obtained from agitated solutions were smaller than those from un-agitated solutions.
An FEM simulation utilizing a dynamic explicit code was conducted to investigate the flange wrinkle behavior of square shell deep drawing of anisotropic steel sheets. In the FEM simulation, anisotropy of the γ-value is taken account of by using the Barlat-Lian '89 equation. The yield locus diagram and the influence of anisotropy were investigated by changing the combination of the R00, R45 and R90 value. One quarter portion of the 75 mm square punch and 80 mm square die was used in the simulation and frictional coefficient μ=0 (Teflon lubricant condition) was adopted. As a result, the greater the m-value of the yield locus, the greater the yield stress (YS) and the greater the ΔCL value (R90-R00)/Rave become, the bigger the flange wrinkle.