The present critical review aims at evaluating long-term creep properties from short-term tests with enough reliability. Two examples cited in the present review are extrapolation of creep rupture lives of short-term creep tests to longer term by means of the Larson-Miller equation and evaluation of a rupture life of an on-going creep test from its minimum creep rate with the aid of a Monkman-Grant relation. Short-term creep tests as short as 1 h are carried out at higher temperatures, and their results are extrapolated toward lower temperature and longer term as long as 1 Mh. A temperature range of the tests can be 200 K. This means that the total variation of effective creep duration is 9 orders of magnitude in terms of diffusion of atoms. In the extrapolation, temperature dependence of rupture life, in other words, a Larson-Miller constant C is assumed not to change from the high temperatures to the low temperature. The Monkman-Grant relation determined by short-term creep tests at higher temperatures and higher stresses is applied directly to the long-term on-going creep test. However, microstructures, creep deformation mechanisms and creep fracture mechanisms of a material cannot be kept unchanged over the wide time range. The difference between the assumption and the reality can result in a large difference between the creep rupture life evaluated and the actual creep rupture life. It will be discussed what should be done for improving reliability of the evaluation.
Flow excitation in a solid-liquid mixed region surrounded by a solid layer is not easy because of difficulty of direct exertion of force on the liquid and large apparent viscosity. Ultrasound is expected as a flow excitation tool in the solid-liquid mixed region because it can transmit force from the outside of the surrounding solid layer. Thus, investigation on the liquid motion under the ultrasound imposition on the solid-liquid mixed region has been done in this study. Because of the large difference of acoustic impedance between an alumina and a polyacetal, they were chosen as the solid particles in the solid-liquid mixed region while water was chosen as the liquid. The superficial velocity in the solid-liquid mixed region increased as its length became shorter under the condition that the voltage on the ultrasound vibrator was constant. Apparent porosity under the imposition of the ultrasound was larger than the real porosity evaluated without the ultrasound when the water-alumina system was used, and the former increased as the solid-liquid mixed region length became shorter. On the other hand, the apparent porosity under the ultrasound imposition was only slightly larger than the real porosity without the ultrasound imposition in the case of the water-polyacetal system. The difference between these systems might be caused by the acoustic impedance difference between the alumina and the polyacetal.
In order to formulate shrinkage behavior of sinter during softening process, the effects of load and reducing gas concentration on the shrinkage rate of sinter packed bed has been investigated using a softening-melting furnace. As a result, the shrinkage rate increased with increasing the load on the packed bed in temperature range from 1270 to 1440 K, and decreased with increasing the reduction degree of sinter in temperature range from 1470 to 1570 K. The results indicate that shrinkage mechanisms vary depending on temperature.
In this study, softening process of sinter was formulated by dividing into two temperature regions. In the region I, the shrinkage rate increased in proportional to the impressed load, and increased in inversely proportional to the softening viscosity. In the region II, the shrinkage rate increased in proportion to the generation rate of melt, and decreased with increasing the volume of metallic iron in the packed bed. The shrinkage in region II was inhibited with increasing the reduction degree of sinter since formed metallic iron served in the role of aggregate. The shrinkage rate of each region was expressed as functions of initial concentration of sinter, load, temperature, and reduction degree. The calculated values were in good agreement with the experimental ones.
Nitrogen gas absorption phenomena in molten steel during bottom bubbling were experimentally studied. Changes of nitrogen in molten steel were observed with 10, 14 and 200 kg scale experiments and it was confirmed that absorption rate of nitrogen were controlled by mass transfer. In addition, bubble dispersions in each experiments were visualized with volume of fluid method (VOF) solver of OpenFOAM and the area of free surface, AS and bubble, AB were numerically evaluated.
The measured data were evaluated with a mixed control model of liquid-phase, gas-phase and chemical reaction. Numerically evaluated AS and AB were utilized for separation from volumetric rate constant of nitrogen absorption, Akm to mass transfer coefficient, km. As a result, mass transfer coefficient at bubble, km,B and free surface, km,S were obtained. In addition, relation between stirring power density, ε ˙ and both km,B and km,S were investigated and it was revealed that km,B and km,S increased in proportion to ε ˙ 0.6.
A tablet made of one of either TiN, MgO or α-Al2O3 was immersed in molten steel containing 0.05 mass%C and undercooling for nucleation of δ-Fe was measured. Undercooling was smaller in order of TiN, MgO, α-Al2O3. Namely undercooling in case of a TiN tablet immersion was the smallest, no more than 2.1 K. This result shows that although disregistry between δ-Fe and MgO is nearly equal to that between δ-Fe and TiN, MgO is obviously less effective heterogeneous nucleation site for δ-Fe than TiN. Chemical term and structural term which were parts of interfacial energies acting for nucleation of δ-Fe were calculated. It is concluded that because chemical term, (γCLch–γCSch)/γSL, in case of MgO is much smaller than that in case of TiN, MgO is less effective than TiN.
A new online rolling model of the draft schedule setup for a plate mill has been developed. This model comprises plate temperature, rolling force function and flow stress calculations and their coupling for the roll separating force estimation. The roll separating force calculation is also used when the work roll gap control is made realising a precise plate thickness control for each rolling pass, which is often referred to as an adaptive control. The temperature model and roll separating force model, as well as its inverse calculation (calculate entry thickness from exit thickness and given roll separating force), are involved in the draft schedule setup calculations. Plate rolling is carried out according to the setup calculation results and thus the product plate quality is largely attributable to the setup calculation preciseness. In this model, a one dimensional finite element model is employed to the temperature calculation that enables a precise temperature control which is necessary for the Controlled Rolling (CR) technology. Another development includes the rolling force function model; a new mathematical model which takes the peening effect into account, derived from the three-dimensional rigid-plastic finite element calculations. Finally, a flow stress model is developed taking into account the metallurgical nature such as work hardening, recrystallization and recovery. The coupling of these models allows to a physical based precise model without unnecessary artificial fitting parameters.
In addition, for eliminating the convergence loop, an attempt has been made introducing a multi thread computing using General Purpose computing on Graphic Processing Unit (GPGPU). Thanks to this parallel computing technique, the computational time was remarkably reduced. The model was installed in a process computer and some trial rolling tests were conducted.
Hot-stamped galvannealed steel sheets (GA) are widely applied to produce automobile components that require high strength. The hot-stamped components tempered for the control of strength improve the collision property. This study investigated the structural changes in the hot-stamped GA coating and the influence on corrosion resistance by tempering. It has been revealed that by tempering between 500-600 degrees Celsius, the Fe-Zn solid-solution phase in the hot-stamped GA coating changed into two phases: the Γ phase and the Fe-Zn solid-solution phase with less Zn, and these changes in the coating structure improved the corrosion resistance. The phase decomposition is considered to result from discontinuous precipitation of the Fe-Zn supersaturated-solid-solution.
SUS316L is generally believed as a stable austenitic stainless steel, but strain-induced martensitic transformation can occur when large deformation is given. The predominant mechanisms, especially, in terms of the effect of grain size have been reported, however, it is not still clear. In this study, strain-induced transformation behavior of SUS316L steel was investigated from the points of grain size and dislocation density. Microstructures with various grain sizes and dislocation densities were fabricated by warm multi-pass multi-directional caliber rolling and annealing. Fully recrystallize microstructures with austenite grain sizes of 12, 18 and 27 µm were fabricated. Bimodal structures with ultrafine grains and coarse grains including high density dislocations were also fabricated. These specimens were rolled at a reduction strain from 0.12 to 2.7 at liquid nitrogen temperature to occur enough amount of strain-induced martensitic transformation. To evaluate martensite volume fraction in these cryogenic rolled materials, transmission type X-ray diffraction in synchrotron radiation of Spring-8 was used. Through thick XRD data can be obtained by this method. Regarding fully recrystallized materials, volume fraction of strain-induced martensitic transformation decreased with decreasing in grain size. It is very clear that gran refinement improves the mechanical stability of austenite. On the other hand, regarding bimodal structure, accumulated dislocations promote transformation, and transformation rates were higher with dislocation density at lower reduction strain area. However, saturated volume fractions decreased with decreasing in grain size. Therefore, strain-induced transformation is affected by both grain size and dislocation density having opposite effects, retarding and promoting the transformation.
Charpy impact value of the case hardening steel subjected to combined heat treatment with excess vacuum carburizing and subsequent induction hardening was evaluated. The purpose of this study is to clarify the effect of retained austenite and cementite on the impact property. The characteristic of combined heat treatment is that the initial microstructure can be designed easily. The initial microstructure is designed by carburizing and annealing at the hyper-eutectoid composition of 1.3 mass% C and subsequent induction heating temperature is chosen between Acm and A1 to obtain different amounts of retained austenite and cementite. The impact value improves by the induction hardening with increasing heating temperature and the tempering. The steel treated at the low heating temperature shows intra-granular fracture irrespective of the presence of pro-eutectoid cementite. As the heating temperature increases, the formation of C solid solution progresses by the decomposition of cementite and increased retained austenite transforms into the deformation-induced martensite by the impact energy, thereby increases intra-granular strength. Hence critical fracture strength transits to grain boundary strength and showed inter-granular fracture at the interface of cementite and matrix in this study. The impact value showed the correlation with the amounts of retained austenite before the test and the decrement in retained austenite before and after the test. The effect of retained austenite is due to the plastic deformation of austenite, the increase of the compressive residual stress generated by deformation-induced martensite transformation, and the consumption of the impact energy as the driving force for deformation-induced martensite transformation.
Blast furnace (BF) dust is recycled as iron source in sintering process. However, dust-recycling rate is limited due to the zinc in dust. Thus, it is necessary to remove zinc from dust in order to improve dust-recycling rate. In this study, zinc-separating process of BF dust using hydrometallurgical (wet-type) system is proposed with a zinc purification process. Laboratory-scale experiments using BF dust were conducted to investigate zinc and iron behavior. This process is composed of (1) acid leaching treatment, (2) leachate-purification process and (3) zinc-recovery process as alkali precipitation. By acid leaching treatment, 68 wt% of zinc was selectively leached into the solution under the pH value of 2.0 with sulfuric acid, while only 6.4 wt% of iron was leached. Leachate-purification process is the key process. The iron leached by acid leaching treatment was completely separated from leachate as iron hydroxide under the pH value of 5.0, added with hydrogen peroxide as oxidizing agent. On the other hand, zinc in leachate was maintained at the same level and well-purified successfully. Zinc was finally recovered as sludge of zinc hydroxide precipitation at alkali condition such as the pH value of 9.0. Zinc concentration in the sludge was concentrated to 42.6 wt%. With this new process, zinc in dust was successfully separated and the sludge containing high concentrations of zinc was obtained. Thus, dust can be potentially used as zinc source as well as iron source.