The bimetallic work rolls are widely used in the roughing stands of hot rolling stand mills. The rolls are classified into two types; one is a single-solid type, and the other is a shrink-fitted assembled type consisting of a sleeve and a shaft. Regarding the assembled rolls, the interfacial creep sometimes appears between the shaft and the shrink-fitted sleeve. This interfacial creep means the relative displacement on the interface between the sleeve and the shaft. This creep phenomenon often causes damage to the roll such as shaft breakage due to fretting cracks. Although to clarify this creep mechanism is an important issue, experimental simulation is very difficult to be conducted. Since few studies are available, in this paper, the interfacial creep phenomenon is simulated by using the elastic finite element method (FEM) analysis. Here, the roll rotation is replaced by the road shift on the fixed roll surface. It is found that the interface creep can be explained as the accumulation of the relative circumferential displacement along the interface.
More than one thousand intragranular and intergranular precipitates of nitrogen-added austenitic stainless steel, SUSXM15J1, were characterized by FIB-SEM serial-sectioning tomography, by conventional transmission electron microscopy (TEM) and by scanning transmission electron microscopy (STEM). All of intragranular precipitates were found nitrided to form dichromium nitride, Cr2N, Two types of intergranular precipitates were found Cr2N and Cr3Ni2Si(N), and some of them were grown and jointed due to the additional heat treatment during the tensile test at 1173 K. which probably contributed to its high-temperature strength.
A composition-modulated Co-Ni laminated plating has been developed to extend the lifetime of molds used for continuous steel casting. We have investigated the relationship between the laminated structure and the mechanical properties of the plating films. The tensile strength of as-plated film increased with increasing thickness of the constituent layers, while the elongation did not depend on the thickness of the constituent layer and remained almost constant in the range from 3 to 5%. Heat treated at 400°C improved the tensile strength and the elongation. The improvement in the elongation was remarkable, and the elongation reached 13% in the film composed of layers with a thickness of 0.8 µm. The layer with low Ni content had an hcp structure, and that with high Ni content comprised two phases of the hcp and fcc structures in the as-plated state. By the heat treatment, the high Ni-content layer turned into the single fcc phase, while the low Ni-content layer kept the hcp phase, and accordingly, the film structure changed into the one where the lamination of the hcp and fcc layers was distinct. The fact that the fcc layers, which was easily deformed, were formed continuously in the lateral direction, was seemed to contribute to the significant improvement in the elongation after heat treatment.
In order to understand the cause of heat-to-heat variation in creep strength of Grade T91 steels, influence of initial microstructure on creep strength has been investigated. Distribution of chromium concentration considered to be a remained segregation was observed, corresponding to a lamellar contrast parallel to longitudinal direction of boiler tube. Standard deviation (SD) of ΔCr was employed as an indicator of degree of segregation, and good correlation was recognized between SD of ΔCr and creep rupture life at 650°C. Remained segregation was reduced by renormalizing heat treatment at not 1250°C, but 1200°C. Creep rupture life of the steel subjected to renormalizing heat treatment at 1200°C and tempering at 760°C, followed by normalizing and tempering with a standard heat treatment condition for Grade T91 steel, was prolonged for 2.3 to 2.8 times. Strengthening effect of renormalizing at 1200°C to reduce a remained segregation was confirmed by creep tests for up to about 10,000h at 600 and 650°C. Decreases in number density of M23C6 carbide particles, length of high angle boundaries and average KAM value during creep exposure have been promoted by a presence of remained segregation. Since diffusion is enhanced by concentration gradient of elements, degradation due to microstructural change is promoted by a presence of remained segregation. Segregation should be reduced to obtain high creep strength with homogenized concentration of chemical composition.
The effect of strain rate on the hydrogen embrittlement property of an ultra high-strength TRIP-aided bainitic ferrite (TBF) steel with bainitic ferrite matrix was investigated to clarify the correlation between the transformation behavior of retained austenite and the hydrogen embrittlement fracture behavior of the TBF steel. Tensile tests were carried out at the strain rates between 5.56×10–6 and 2.78×10–2 /s without and with hydrogen charging. Hydrogen analysis after tensile tests was conducted by using thermal desorption spectroscopy (TDS). Fracture strain decreased with decreasing the strain rate due to the hydrogen absorption to the TBF steel although fracture strain without hydrogen charging slightly increased with decreasing the strain rate. However, it was observed that transformation behavior of retained austenite was hardly changed by the hydrogen absorption and the change in the strain rate. When tensile test was carried out to the TBF steel at the slow strain rate with hydrogen charging, fracture surface of quasi cleavage fracture containing flat facet, which was fractured transformed martensite, was obtained and the crack perpendicular to the tensile direction was observed near transformed martensite. It was considered that the decrease in the resistance to hydrogen embrittlement of the TBF steel tensile tested at the slow strain rate was attributed to the initiation of flat facet and the hydrogen concentration at the crack tip due to the hydrogen diffusion from transformed martensite during tensile testing at slow strain rate.
The roles of solute C and the grain boundary in the strain aging phenomenon of polycrystalline ferritic steel were investigated using Nb-bearing ULC steel sheets with a relatively low solute C content of 1-3 mass ppm and ferrite grain sizes of 9.5 μm and 183 μm at aging temperatures from 70 to 400ºC. The steels exhibited two definite hardening stages. The first hardening stage appeared in both fine- and coarse-grained specimens, in which the increase in YP (ΔYP) became saturated at around 30 MPa. From the apparent activation energy and hardening kinetics, the hardening mechanism was assumed to be dislocation pinning by solute C atoms. The second hardening stage, significantly appeared in fine-grained specimens accompanying a large increase in the Hall-Petch coefficient; ΔYP was quite large, reaching 90 MPa. Fine precipitates were not detected in aged specimens observed by TEM and 3DAP. Segregation of solute C to the grain boundaries and diffusion of Fe atoms in the grain boundaries were proposed as possible mechanisms of this second hardening. Grain-boundary hardening is assumed to be one of the hardening mechanisms in the strain aging of the polycrystalline ferritic steel.
The peel strength of 16Cr-ferritic stainless steel / aluminum rolled clad sheet is improved by heat treatment at 300 to 500 °C after clad rolling. The joint mechanism was studied by analyzing the fracture surface after the peel test, taking into account the characteristics of the peel test method.
The detailed investigation into the fracture surface after peel test allows us to classify fracture sites into three categories: 1) ductile fracture in aluminum base material, 2) flake-like fracture in 16Cr-stainless steel, and 3) fracture in the intermediate interface layer between aluminum and 16Cr-stainless steel. It was revealed that the intermediate layer breakage mainly occurs in the as-rolled clad sheet; whereas after heat treatment at 300 to 500°C, the aluminum base material breakage predominantly occurs.
Close observation into the state of deformation at the crack tip of the peel tested specimen leads us to conclude that clad rolled and subsequently annealed aluminum undergoes plastic deformation during the peel test because significant softening easily takes place preferentially in aluminum during low temperature heat treatment. This might imply the decrease in peel strength by heat treatment, which is different from the fact. However, the load area responsible for peeling force during the peel test is inferred to substantially increase, which gives rise to the increase in peel strength. This is because the peel strength is simply evaluated by dividing the peel force by the width of the specimen.
Steelmaking slag has been used in the coastal area for the purpose of the environmental improvement. Short-term alkali elution behavior induced by free CaO in the steelmaking slag was investigated by a batch and continuous vessel. In this study, long-term alkali elution experiments were done in an open channel vessel with a slag box to make clear the decreasing mechanism of the alkali elution from steelmaking slag into seawater. The experimental period was 44 d (1056 h). The pH values increased just in the beginning of the experiment, and then gradually decreased in about 100 min. They showed almost the same value as that of seawater in 1200 min. As the experimental time passed, the white deposits on the steelmaking slag layer, which were composed of Mg[OH]2 and CaCO3, spread to the lower slag zone in the slag box. The white deposit zone of larger slag size was more rapidly diffused downward than that of smaller one due to larger voidage in the slag layer. While carrying out the batch test with the used slag by the long-term experiment, the alkali elution rate of slag layer on the open channel flow was 0.03-0.11 times decreased compared with the unused slag, whereas that of the lower positioned slag layer was 0.10-0.38 times decreased. From these results, it was found that the alkali elution rate was reduced by the white deposit on the steelmaking slag.
In steelmaking slag, phosphorus is distributed between liquid phase and solid solution of 2CaO·SiO2 and 3CaO·P2O5 at refining temperature. Using the difference in water solubility of the solid solution and the other phases, we are developing the process to separate phosphorus by acid leaching. In this paper, the optimum condition of leaching by nitric acid by the control of valency of Fe in slag was investigated and the recovery of phosphorus from leachate was also conducted. To dissolve the solid solution, pH has to be decreased to 3 and the solid solution containing FeO showed lower dissolution ratio. To suppress the dissolution of other phases in slag at this pH, slow cooling to avoid the formation of glassy phase was necessary. The artificially steelmaking slag with the appropriate composition was made and subjected to the leaching experiment. The dissolution ratio of phosphorus has reached about 91% and the phosphorus content in the residue was enough low. After the separation of residue, pH of the leachate increased to precipitate phosphate. At pH=7, 80% or more of the phosphorus in the leachate was precipitated and the phosphate content of the precipitate was about 25% after calcination.
The Japanese nails had been produced by the traditional ironmaking way of Tatara until the Edo period since the late of 6 Century. The Tatara is characterized as the ironmaking using very fine iron sand as the resources and is distinguished from the pre-modern ironmaking way in Europe using lump iron ore. The quality of the Japanese nails is affected from steel produced by Tatara in each age. The 14C ages of 3 Japanese nails were measured with accelerator mass spectrometry and calibrated to calendar years. Each 14C age provided plural calendar year periods with definite probabilities, and one of the periods determined in comparison with the history of temples and their repair records. The production ages of nails used in the Daibutuden of the Todaiji temple, the living quarters of the Manshuin temple and the Zaoudo of the Yoshino-Kinpusenji temple are before 1692, the 12 Century and before 1592, respectively, when they were repaired or reconstructed.