Precise control on inclusions is of great importance for improving steel quality. In secondary refining, three types of inclusion are generally observed in Al-killed steel: Al2O3, MgO·Al2O3 spinel, and CaO–Al2O3 type. Many researchers have reported on inclusions transformed in the routine of Al2O3 → MgO·Al2O3 spinel → CaO–Al2O3 during secondary refining in Al-killed steel. The deoxidizer of Al is intentionally added to the steel, and Al2O3 inclusions are formed as a deoxidation product. However, MgO·Al2O3 spinel and CaO–Al2O3-type inclusions have been observed even without any intentional Mg or Ca addition. Therefore, it is important to clarify the source of Mg and Ca causing MA spinel and CaO–Al2O3-type inclusions formation, in order to control the compositions of the inclusions. Regarding to this phenomenon, a good review was published by Park and Todoroki in 2010. Several studies have since been conducted. This paper summarized the research activities on the composition changes of inclusions during secondary refining from the aspect of thermodynamics and kinetics.
Crystal plasticity models enable predictions of macroscopic deformation behavior as well as texture evolution of metallic materials based on mesoscopic deformation at the grain level. Owing to recent improvements in predictive accuracy, crystal plasticity models are expected to be used not only for academic purposes but also for industrial applications. There are several possible approaches for utilizing crystal plasticity models in industrial applications, including numerical material testing, in which the material parameters of phenomenological constitutive models are determined; alternative constitutive equations in simulations; and the development of innovative materials with improved formability. In this review paper, recent progress in crystal plasticity modeling, specifically in terms of engineering applications, is discussed. The focus is primarily on hexagonal close-packed (hcp) metals, including magnesium alloy and commercially pure titanium sheets, which exhibit strong anisotropic and asymmetric deformation behavior. On the basis of our recent progresses, the crystal plasticity modeling was first explained, followed by some application examples for a variety of loading conditions, including uniaxial tension and compression, reverse loading, and biaxial tension. The application to face-centered cubic (fcc) and body-centered cubic (bcc) metals and future prospects are also discussed.
Recovery of multisource metallurgical wastes can facilitate the recovery of valuable metals, and achieve the full utilization of slag. Recycling nickel slag by aluminum dross with converter-slag addition was studied. Based on the element mapping of slags, mineral phases in the modified slag were reconstructed under the interaction of nickel and converter slags, and ‘FeO’ could be separated from the relevant structures of the two slags. Element mapping and chemical analysis of the metal phase after reduction indicated that the reduced product was Fe–Cu-based alloy, element mapping and XRD detection of the secondary slag indicated a complex characterization. The influence of factors, including the basicity of the modified slag, the reduction temperature and the Al/‘FeO’ ratio on the recovery degree of Fe, Cu, Mn and P, was discussed and the optimal technical parameter was determined as 1.0, 1773 K and 0.67, respectively.
Molten silicate slags with transition metal oxides roles the accelerated oxidation and decarburization of steel. In 1978 to 1990, Sasabe et al. measured the flow rate of oxygen permeating through molten slags and realized the marvelous increase of the flow rate by 1010 times by the addition of only 0.2 mass% iron oxide to molten silicate slags. The permeability coefficient of oxygen of molten slags as a physical property is calculated from the flow rate of oxygen. The marvelous increase is interpreted as oxygen transfer through slag like a ball thrust, that is caused by the redox reaction of oxygen at both surfaces and the move of all ions in slag under electric field generated by oxygen potential gradient.
In this study, the effects of the addition of Ce and Ti on the evolution of inclusions in Al deoxidized Fe-17Cr-9Ni austenitic stainless steel were investigated. When the Ce was added before Ti, Al2O3 can be effectively modified into Ce-containing oxides by Ce. With the increase of Ce content, the oxides evolutionary process is as follows: Al2O3 → CeAl11O18 → CeAlO3 → Ce2O3. After the addition of Ti, TiN appeared on the surface of oxides except Ce2O3. When Ti was added before Ce, due to the simultaneous effects of Ti on the modification of smaller size Al2O3 and precipitation of TiN, spherical Al2O3–TiOx inclusion that was completely wrapped by TiN formed. On the other hand, the larger size Al2O3 was not significantly modified by Ti, and cannot completely be wrapped by TiN. After Ce was added, the oxides which were not completely wrapped by TiN were modified to Ce-containing oxides. However, Ce could not modify the oxides that were completely wrapped by TiN.
The effect of TiO2 content ranging from 0 wt% to 4 wt% on the viscosity and sulfide capacity of CaO–SiO2–MgO–Al2O3–BaO–TiO2 slag system was investigated using the rotating spindle method and gas-slag equilibration technique, respectively. The measured sulfide capacity was further compared with the predicted values by some empirical models based on the corrected optical basicity when the effect of charge compensation effect of Ba2+ and Ca2+ was considered. The results show that the viscosity declines with TiO2 content increasing while keeping CaO/SiO2 ratio and other components contents constant, and TiO2 seems to be more effective in decreasing viscosity when polymerization degree of slag is higher. The sulfide capacity decreases with an increase in TiO2 content although the dynamic condition is improved due to the viscosity decreasing. The sulfide capacity is enhanced with the increasing corrected optical basicity and it can be predicted by using the modified predicted values based on Zhang’ model.
Understanding chemical structure of primary coal tar is substantially important to promote advanced application and utilization of coal where chemical reaction plays a key role. Mass spectrometry allows us to characterize complex mixture like coal tar pitch, coal extractions, and heavy oil. In the present study, two kinds of primary coal tar A and B were prepared from different types of coal A (bituminous coal) and B (subbituminous coal). The main components such as Hexane Soluble (HS) and Hexane Insoluble-Toluene Soluble (HI-TS) fractions obtained by extraction of each tar were measured by Field Desorption Mass Spectrometry (FD-MS) to determine their constituent chemical structures. The 1H NMR spectra of HS and HI-TS were measured to confirm the reliability of proportion of hydrogen types that have been determined by FD-MS. As a result, both of the proportions in HS fractions were found to be almost consistent, while those in HI-TS fractions show a difference because of their more complex chemical components than HS fractions. Based on the combined analysis of FD-MS and NMR, we concluded that the HS in primary coal tar A mainly consists of polycyclic aromatic hydrocarbons (PAHs) and aliphatic structures including aromatic carbons, while the HS in primary coal tar B mainly consists of aliphatic structures. These results suggest that determining chemical structures and their ratios only from FD-MS spectra is useful to concretely clarify difference between some types of primary coal tar.
Classification of chemical structures contained HS fractions for primary coal tar A and B.
The influence of TiO2, binary basicity and Al2O3/TiO2 ratio on the heat capacity, enthalpy and slag fluidity of CaO–SiO2–MgO–Al2O3–TiO2-based slag at 1693 K, 1723 K, and 1753 K (1420°C, 1450°C, and 1480°C) was investigated in this work. From the calculation results, it was found that the heat capacity of the slag increased with the increasing of TiO2 content and the Al2O3/TiO2 ratio and with the decreasing of basicity in the experimental temperature range. Enthalpy change increased with the increasing Al2O3/TiO2 ratio and the decreasing of TiO2 content. With the increasing of basicity, the slag temperature rises slightly and the viscosity decreases along with it. Additionally, the larger the basicity, the smaller the viscosity fluctuation under heat decrement. Therefore, a proper increase in basicity contributes to reduce viscosity fluctuation, for the current slag system, the appropriate basicity should be 1.15–1.20. Besides, the fluctuation of the temperature reaches a small value around the Al2O3/TiO2 ratio is 0.6, and in metallurgical production, the heat input should be adjusted in time according to titanium content charge fluctuations, thus ensuring good fluidity and an adequate reaction between the slag and metal. The above experimental results can provide a reference for ironmaking enterprises using more vanadium–titanium magnetite ore.
In this work, the influences of moisture content of coal on the structure and reactivity of cokes were investigated by blending different proportion of dry coal (with < 2 wt.% moisture) and wet coal (with ~10 wt.% moisture) and analyzing the gasification of the produced coke. The results indicated the coke formed from dry coal has the highest specific surface area and thinner pore walls. The results of isothermal thermogravimetric method show that the order of gasification reactivity of bulk coke from different proportion of wet coal is: 0 wt.% wet coal, 100 wt.% wet coal, 60 wt.% wet coal and 30 wt.% wet coal. In order to eliminate the influence of diffusion on the gasification reaction, coke with a particle size fraction of less than 48 µm was used for the non-isothermal gasification reaction. Results show that the gasification reaction curves of four samples are similar in the gasification process. It was concluded from kinetics analysis that the volume reaction model is well fitted with the experimental data. The activation energy with the volume reaction model is 191.9, 203.1, 190.1, and 190.8 kJ/mol. It was concluded that the moisture content of coal has little effect on the activation energy of the gasification, while the coke gasification kinetics is mainly determined by the coke pore structures which influence reaction surface.
To better understand metallurgical coke behavior in blast furnace, the preparation of coke analogues was improved by using demineralized coke powder. Scanning electron microscope, Raman spectroscopy, mercury intrusion method, and CO2 gasification reactivity test were used to establish the representativeness relation between different coke analogues and metallurgical cokes. The results show that the coke analogues prepared by graphite powder and demineralized metallurgical coke powder were in general representative of industrial coke. With the controlled and reproducible pore characteristic, the average pore diameter of coke analogues is smaller and the average pore area is bigger, while the pore connectivity of metallurgical coke is stronger. Analogue prepared from demineralized coke has a slight superiority over that prepared from graphite in carbon structure and gasification reaction with CO2. In general, the coke analogue made from demineralized coke has higher comparability to industrial metallurgical coke and are more suitable for laboratory research.
A computational model coupling electromagnetic field with a macroscopic heat and fluid flow is developed to investigate the flow pattern and solidification in a vertical continuous caster using a four-port submerged entry nozzle (SEN) with mold and strand electromagnetic stirring (M-EMS and S-EMS). The flow pattern and solidification features of the bloom strand without and with EMS in the caster using the four-port SEN is analyzed and compared with that using a straight-port nozzle. The effects of the stirring parameters and the position of the strand stirrer on the flow and solidification are discussed. The approach to identify the optimum stirring parameters by the comparison of tangential velocity is suggested. The results show that the application of M-EMS in a four-port SEN can weaken the strength of the jet impingement from every port of the four-port SEN, and rapidly dissipate the superheat of the melt and reduce the liquid fraction in the mold. In spite of inhomogeneous shell growth in the mold, the swirl velocity obtained by a four-port SEN and M-EMS and the solidus fraction by S-EMS is above those of a single-port SEN with the same stirring strength, which is favorable for the formation of more equiaxed crystals. For the S-EMS, the solidified shell thickness is the main factor to determine the stirring position and the tangential velocity at the same stirring intensity. In terms of different ported SENs, it is necessary to perform specific optimization of the stirring parameters of M-EMS and S-EMS.
In order to assess the wear damage of the lining refractory in the RH degasser, a transient 3D numerical model has been established using volume of fluid approach-discrete phase model (VOF-DPM) technology. The gas-oil-water three-phase flow in a RH degasser water model was evaluated. The breakup and coalescence of gas bubble was taken into account, and moreover the bubble diameter changed with static pressure. The wall shear stress and turbulence intensity were employed to predicate the erosion rate of the lining refractory, while the diffusion coefficient of the refractory material and the slag property at high temperature were used to consider the corrosion rate. The effects of the operational parameters on the refractory wear rate were clarified. A careful comparison between the experimental and the numerical results was conducted for the model validation. The results show that the wear behavior of the lining refractory at the up snorkel wall is the most severe due to the rapidly rising bubble. The vacuum chamber wall that near the up snorkel is also subjected to a serious wear damage. Besides, a higher wear rate is observed at the ladle wall that close to the oil/water interface, since both the physical erosion and chemical corrosion contribute to the wear damage of the lining refractory here. The developed model could help smelters to estimate the remaining thickness of the refractory in the RH degasser under different operational conditions.
The inclusion characteristics and microstructure in the low carbon microalloyed steel with addition ZrO2 nanoparticles were investigated by high temperature experiment and metallurgical analysis. The results showed that after ZrO2 nanoparticles were added, a large number of Zr–Al–Si–O+MnS inclusion, ZrO2+Al–Si–O+MnS inclusion, and ZrO2+MnS inclusion appeared in the steel, and these Zr-containing inclusions were effective to induce acicular ferrite (AF) formation. With the amount of added ZrO2 nanoparticles increased from 0.013% to 0.054%, the inclusions types had no significant effect, but the inclusions size distribution, number density and average diameter were affected. When the addition amount of ZrO2 nanoparticles was 0.027%, the proportion of large inclusions (larger than 5.0 µm), the inclusions number density and average diameter all were reached the extremum values, respectively 10.81%, 111/mm2, 2.62 µm. Moreover, as ZrO2 nanoparticles adding into steel, the majority solidification microstructure changed from bainitic ferrite (BF) to AF. With the adding amount of nanoparticles increase, the proportion of AF first increased and then decreased, also reached the maximum of 67.65% as the addition amount was 0.027%. Finally, Mn-depletion zone (MDZ) in the vicinity of Zr-containing inclusion was observed, and the MDZ was believed to be one of the possible mechanisms of Zr-containing inclusions inducing AF formation.
As an efficient stirring method, bottom-blowing technology was applied in the present electric arc furnace (EAF) steelmaking process to improve the dynamic conditions of the molten steel. This article describes the development of a numerical model to simulate the 3D multiphase flows (gas, steel, and slag). Comparisons among uniform and non-uniform (linear and triangle distributions) bottom-blowing gas rate arrangements were performed with both metallurgic and dynamic parameters obtained from the numerical simulation process and separated liquid steel analysis. The numerical simulation results indicated that the bottom-blowing scheme with the gas rate in a linear change distribution had the best stirring effects in the molten bath. In addition, the dynamic conditions in the molten bath were worse when the bottom-blowing gas rate change was focused on the nozzle near the eccentric bottom tapping area. Furthermore, water model and industrial experiments were performed. For this purpose, 120 sets of heat industry data were collected in the 100 t EAF steelmaking process. The results showed that the non-uniform bottom-blowing scheme is more able to improve the dynamic conditions of the molten bath compared with the conventional uniform gas rate distribution, which further validated the reliability of the present numerical simulation results.
The occurrence of longitudinal surface cracks in hypo-peritectic carbon steel slabs depends largely on the cooling capacity of the mold and the flow velocity of molten steel below the meniscus. The influence of both flow velocity of the molten steel below the meniscus and the heat flux in the copper mold were examined using continuous casting tests and numerical simulation of the molten steel flow. The casting speed was fixed, and the meniscus flow velocity was controlled by adjusting the port size of the submerged entry nozzle. The molten steel flow velocity was predicted by a three-dimensional unsteady-state numerical simulation. Heat flux in the copper mold was calculated based on temperature readings from thermocouples arranged in the direction of both the mold width and mold length. When the difference in flow velocity of molten steel in the mold width direction became large, longitudinal surface cracks occurred in the central region of the slab. In these cases, the heat flux below the meniscus in the mold width direction was not constant. Small holes were drilled along the central region of the mold width. This decreased both the heat flux and tensile strength of the central region of the slab width, and successfully reduced the occurrence of longitudinal surface cracks.
This research is aimed to correlate heat transfer coefficient to pressure at the interface between 19Cr-14Mn-0.9N high nitrogen steel cylindrical ingot and cast iron mould, during the pressurized solidification process of cylindrical ingot. The correlations were obtained by mathematical inverse model of heat conduction problem. Validation results indicate that this model is applicable to investigate the change in interfacial heat transfer coefficient during the pressurized solidification process of 19Cr-14Mn-0.9N high nitrogen steel, and guarantee the correlation accuracy. Combing with theoretical derivation for cylindrical steel ingot, the change in interfacial heat transfer coefficient with time can be described by hf,0.5 = 679.68t−0.12 W/(m3·K) for 0.5 MPa, hf,0.85 = 753.53t−0.12 W/(m3·K) for 0.85 MPa and hf,1.2 = 790.39t−0.12 W/(m3·K) for 1.2 MPa, quantitatively. Meanwhile, an empirical formula was presented to correlate interfacial heat transfer coefficient to pressure, which can be taken as heat transfer boundary to simulate the change in solidification state of 19Cr-14Mn-0.9N high nitrogen steel cylindrical ingot with pressure.
With the strict standards for steel quality and high production rates, the demand for faster and more convenient slag composition analysis for both electric arc and ladle furnaces has become a major issue in industrial steel plants. To overcome the time-delay between slag sampling and results of the slag composition analysis, an on-line slag composition analysis is required. Such a method that can be used in on-line analysis and is also chemically sensitive to the slag composition is optical emission spectroscopy. In this work, the optical emissions from the arc have been measured in an industrial ladle furnace and used for slag composition analysis. This article focuses on CaF2 and MgO, since the CaF2 is a common additive material in the ladle treatment and high MgO content means that the ladle refractory lining is dissolving into the slag. The analysis has been carried out by comparing emission line ratios to the XRF analyzed ratios of CaF2/MgO and MnO/MgO, respectively. The results show that several atomic emissions lines of calcium, magnesium, and manganese can be used to evaluate the CaF2/MgO and MnO/MgO ratios in the slag. It was found out that the plasma temperature derived from Ca I emission lines has a non-linear relation with the CaF2 content of the slag. Additionally, the dissociation pathways of molecular slag components were determined and studied in different plasma temperatures with equilibrium composition computation in order to determine the relations between the slag and plasma compositions.
The industrial application of nanofluids had been explored by many researchers since nanofluids were proposed. However, there were different opinions on the effect in jet cooling. In this paper, 0.4 vol%, 0.8 vol%, 1.2 vol%, 1.8 vol%, 2.4 vol% Al2O3-water, TiO2-water, SiO2-water nanofluids and pure water were used as quenching coolants to complete single jet cooling experiments on the free surface of 50 mm high-temperature steel plate. The results showed that using low concentration (0.4–1.2 vol%) nanofluids could significantly improve the maximum heat fluxes, cooling speed peaks, and moving velocities of peaks along the thickness direction compared with pure water. However, the cooling uniformity in the horizontal direction was reduced, especially with high concentration nanofluids (≥1.8 vol%). Through comprehensive comparison, when 1.2 vol% Al2O3 + water was used as coolant, the optimal cooling efficiency could be achieved, and cooling speed peaks along the thickness were 8.14%–19.70%, 2.16%–3.48% and 0.74%–1.44% higher than that of pure water respectively.
A novel and efficient simulation technique for the purpose of optimization of vacuum-carburizing process was proposed. This method consists of three steps: calculation of gas convection and diffusion, calculation of only gas diffusion, and calculation of carbon diffusion in steel. The first step provides the gas convection velocity that is employed in the second step. Adsorption rate of carbon on the steel surface is obtained in the second step, and carbon concentration in the steel is calculated in the third step based on the adsorption rate of carbon.
Experiments were conducted to verify the proposed method in both laboratory- and industrial-scale reactors. Comparison of the computational predictions to the experimental data revealed that the proposed simulation technique enabled accurate prediction of the adsorption rate of carbon on the steel surface at various temperature conditions, the amount of carburized carbon at each operating time, and the profile of carbon concentration in the steel that is, in other words, the carburized depth. In addition, the calculation of the industrial-scale reactor, whose simulation model consisted of approximately seven million computational meshes, was completed within about two days. Therefore, the proposed simulation technique could be used to control and optimize the process in industrial vacuum-carburizing reactors.
Distributions of mass fraction of C2H2 (colours) and adsorption rate of carbon on the surface of workpieces (mono colour). Additionally, some velocity vectors of the gas are plotted.
The transformation behavior and microstructure in a medium-carbon bainitic steel were investigated by combination of metallography and dilatometry. The fine micro-structural units of carbide-free bainite in non-ausformed and ausformed materials were measured by a transmission electron microscope. Mechanical stabilization of austenite in deformed material and its effect on property were analyzed by nanoindentation and tensile tests. Ausforming with a strain of 0.2 at 573 K can not only accelerate bainite transformation, but also improve the comprehensive properties. The strength and ductility of nanostructured bainitic steel can be simultaneously enhanced by ausforming, which should be attributed to the refinement of bainite and the enhanced volume fraction of retained austenite. Compared to the non-deformed material, the mechanical stabilization of austenite can be optimized by ausforming, resulting in good transformation-induced plasticity effects. Also a very important advantage was that, the bainite transformation time could be minimized into practical scale by prior ausforming compared to traditional low-temperature austempering.
Subjected to quenching processing, the samples of weld heat affected zone containing microstructures of martensite in high strength pipeline steel were simulated and prepared. Effects of electropulsing treatment on the corrosion resistance of simulated samples were studied through electrochemical detections and immersion corrosion experiments. It was found that the corrosion resistance of pipeline steel decreased sharply due to the high lattice strains/dislocation densities and residual tensile stress developed after martensite transformation by water quenching. Interestingly, treated by electropulsing the corrosion resistance of simulated weld heat affected zone samples was dramatically improved, and even exceeded that of the base metal when the current density achieved 5.2 kA/mm2. After electropulsing treatment, the dislocation density and residual stress of the investigated samples were reduced largely, and the rust layer generated after corrosion was more compact, so that its corrosion resistance was improved.
This study investigated the effects of Ni addition on the corrosion resistance of steel in subtropical seashore environments. Carbon steel and 3, 5, and 7% Ni steels were exposed in such an environment for a year. Addition of Ni depressed the corrosion rate of steels and number of cracks in the rust layer. Quantitative and three-dimensional measurement of the cracks with a wide range of widths and volumes in the rust layer was carried out for the exposed steel specimens using the mercury intrusion method. The total crack volume in the rust layers on 5% Ni steel was 60% lower than that for the carbon steel. It is considered that rust layers with less crack volume suppressed Cl– migration through the rust layer. The Cl concentration near the metal interface was relatively lower in the 5% Ni steel by EPMA analysis. And the rust layer on 5% Ni steel also showed a higher permeation resistance than that formed on carbon steel. Considering the formation of rust layers with less volume crack on Ni-added steel based on Morcillo’s model, it is concluded that the Ni addition promoted the formation of a-FeOOH and suppressed the reduction of γ- and β-FeOOH, thus resulting in a more intact rust layer.
The use of alkaline electroplating baths is the essential requirement to deposit Cu directly onto steels because of non-adherent Cu formation by replacement reaction between Cu2+ and Fe in acidic solution. For the development of such an electroplating bath, complexing agents to form soluble Cu complex in alkaline pH is necessary at first. Secondary, the soluble Cu complex must be reduced electrochemically. Cyanide-based baths meet these requirements, but the bath is toxic. In this study, the survey of complexing agents revealed that citric and tartaric acids form soluble copper complex solutions in alkaline pH, and electroplating is possible. The cathodic current density range to obtain smooth and adhesive electroplating with citrate complexed bath was extensive than that with a tartrate bath. It was found that 0.1 mol dm−3 CuSO4 - 0.5 mol dm−3 citric acid baths with pH of 9–11 are optimum to obtain adhesive and uniform Cu layer. Copper electroplating with an acidic CuSO4–H2SO4 bath was possible on 1 µm Cu layer with the alkaline citrate bath. Because the plating rate is high with the acidic bath, the multilayer Cu electroplating from the citrate bath and then an acidic sulfate bath gives a reasonable way for Cu coating onto steels. Elongation test of the steel sheet electroplated with the multilayer Cu showed that detachment of the Cu layer was limited in the vicinity of the broken part of the sheet. It is concluded that the toxic cyanide Cu plating bath can be replaced with a citrate bath.
Steel coated with zinc and zinc alloys is widely utilized for home appliances, construction, automobiles, and other applications due to its high corrosion resistance. In this work, the corrosion behavior at cut edges of Zn–11%Al–3%Mg–0.2%Si-alloy-coated steel sheets (SD) was investigated with a cyclic wet–dry corrosion test. The results showed that SD has anticorrosive property superior to that of zinc-coated steel sheets (GI) during the early period of corrosion. GI produced red rust, whereas SD produced no red rust. After the cyclic wet–dry corrosion test, zinc-containing white rust was deposited on steel substrate. In the case of SD, magnesium reached the center of the cut edge, and a larger area on the steel was covered with white rust. Polarization measurements of steel substrate on which white rust was deposited clarified that the white rust of SD reduced both the anodic and cathodic current densities of the steel substrate more than GI. In the case of SD, the galvanic current between the steel substrate with white rust and the coating layer was small compared to that in the case of GI. It is suggested that this anticorrosive property of SD is caused by magnesium-containing white rust.
A new surface modification technique, called “iron-powder pack (IPP) treatment”, has been proposed. During IPP treatment, carbon and nitrogen diffuse into a metal substrate by treating it with a mixture of iron and carbon powders at high temperature in a nitrogen flow. This paper describes the effects of adding alumina to the mixture of iron and graphite powders on the microstructures and hardness of SUS430 ferritic stainless steel after IPP treatment. The alumina powder was added to prevent powder mixtures from sintering. A 7:3 (volume ratio) mixture of iron and graphite powders was used as a “base powder”, and the volume ratio of the alumina powder added to the base powder was varied from 0:1 to 50:1. A steel pipe packed with SUS430 steel and powder was heat-treated at 1273 K for 3.6 ks in a nitrogen flow and then rapidly cooled in water. A surface-modified layer formed on the SUS430 steel through IPP treatment. When IPP treatment was performed using only the base powder, carbon was detected in the surface-modified layer. Both carbon and nitrogen diffused when alumina was added to the base powder. However, the thickness of the surface-modified layer gradually decreased from 380 to 125 µm with an increase in the amount of alumina. A similar tendency was observed in the surface hardness of the steel. In addition, the effects of subzero treatment and immersion in an aqueous nitric acid solution on the modified steel are discussed.
In this study, a new method for predicting carbon and nitrogen contents of a carbonitrided surface using computational thermodynamics with Thermo-Calc was developed. The nitrogen content of alloyed steel, which is in equilibrium between the steel surface and the atmosphere, was predicted using the nitriding potentials and Thermo-Calc, and the experimental and calculated results were compared using pure iron. For lower nitrogen levels, the accuracy of prediction was sufficient. However, for higher nitrogen levels, the experimental nitrogen content was lower than the calculated value, which was attributed to pore formation. Through a comparison of the described method with the conventional one, it was confirmed that our novel prediction method exhibits sufficient accuracy to predict the nitrogen content following carbonitriding.
The creep strength of 5Cr-0.5Mo steel was determined at 600°C and 78–170 MPa, as well as its relation to the microstructural changes during the creep tests. The microstructural characterization showed that the creep tests were conducted under the presence of a mixture of both intergranular and intragranular M7C3 and M23C6 carbides dispersed in the ferrite matrix. The n exponent of Norton-Bailey law suggested that the creep deformation process occurred through the ferrite grains, which conducted to a transgranular ductile- fracture mode after creep testing. The creep strength of this steel is directly related to the average radius and number density of carbides present during the test. The ferrite grain size of 5 µm seemed to cause an enhancement of the creep strength for this steel in comparison to that of other similar steels reported in the literature.
The deformation mechanism of Fe-20Mn-0.6C twinning-induced plasticity (TWIP) steel was studied with respect to different strain rates ranging from 10−4 to 103 s−1. Moreover, the microstructure of the ultra-high strength TWIP steel at each strain rate was characterized by transmission electron microscopy (TEM). The TWIP steel exhibits three distinct strain hardening stages with increasing true strain. In stage II, dσ/dε shows a plateau at the strain rates of 10−3 to 10−1 s−1, while dσ/dε continuously decreases in the other stages with increasing strain rate. The deformation mechanism of TWIP steel under the high strain rate was a process in which the deformation twin and the dislocation slip promoted and restricted each other. When the strain rate is higher than 102 s−1, the increase in the adiabatic heating temperature (approximately 143°C) suppresses the secondary twinning and enhances the softening effect.
Butterfly martensite grains formed in medium-carbon steel consists of two coarse grains colliding with each other. The present study investigates the relationship between the wing angle of the butterfly and its substructures. The substructures (especially the arrangement of twins in butterfly wings) were observed using transmission electron microscopy (TEM). The observation results show that twins pile up along the longitudinal or width direction in a butterfly wing. This result indicates that butterfly martensite grains can be classified into three patterns by the twin piling direction and that the wing angle of the butterfly is determined depending on the substructure patterns. We also proposed a method to estimate the substructure patterns using data from the electron backscatter diffraction measurement without TEM observation. In the most common butterfly grains, twins pile up in the longitudinal direction in one wing and in the width direction in the other wing, and the wing angle is obtuse. The outer interface of the butterfly wings and the variant combinations of the butterfly wing pair are also investigated. The orientation of the outer interface is determined depending on the substructures, and a strong variant selection in the butterfly wing pair is observed despite the variety of substructures.
Digital image correlation was applied to analyze the strain distribution and deformation-induced martensitic transformation of retained austenite in a low alloy transformation-induced plasticity (TRIP) steel plate under tension. The distribution of strain instilled by tensile deformation was inhomogeneous at a microscopic scale. Strain generated by deformation-induced martensitic transformation was successfully visualized and it led to a homogeneous strain distribution. The retained austenite in the high strain region transformed to martensite preferentially, which demonstrates that inhomogeneous strain distribution affects the stability of retained austenite. The high resolution strain distribution exhibited that a certain amount of strain instilled into retained austenites and there are a lot of strain concentration sites at ferrite/austenite interfacial boundaries in high strain region. Therefore, the stress concentration at the ferrite/retained austenite interfacial boundary occurs due to the difference of strain between the ferrite matrix and retained austenite. These strain accumulation in a retained austenite and/or stress concentration at ferrite/retained austenite interfacial boundary may induce martensitic transformation in high strain regions.
εxx strain distributions strained at 293 K (a)–(d) and 193 K (e)–(f): (a) 0, (b) 0.04, (c) 0.08, (d) 0.12, (e) 0, (f) 0.04, (g) 0.08 (h) 0.12 strains. (Online version in color.)
We have constructed an automatic in situ observation system for monitoring the behavior of small fatigue cracks at the microstructural level that, when used in conjunction with a digital-image correlation (DIC) technique, permits the continuous and automatic tracking and recording of microscopic deformation behavior. To verify the effectiveness of this system, we applied it to the evaluation of small fatigue cracks in heat-treated low-carbon steel. The results confirmed that our system can be used in the automatic tracking and recording of the initiation and early growth behavior of microstructurally small fatigue cracks. By the use of DIC analysis, we also succeeded in visualizing the opening-and-closing behavior of small fatigue cracks as well as the behavior of microscopic microstructural deformations, such as inhomogeneous strain concentrations, that caused the fatigue cracks. Although the early-stage growth of fatigue cracks propagates faster than that of long cracks, it is consistent with long-crack data if the effective stress intensity factor range ΔKeff which calculated by crack opening stress measured by DIC is used.
By using a steel with standardized chemical composition and conventional manufacturing processes for flat-rolled steel strip, a 1500 MPa class stainless steel sheet, whose product of yield strength (YS) and total elongation (El) exceeds 30000 MPa%, was developed and its mass production was established. Besides the excellent YS–El balance, the developed steel sheet has excellent performance for not only an anti-secondary work embrittlement but also high cycle fatigue endurance.
Core technology of the developed method is composed of a combination of high precision cold-rolling and isothermal partitioning treatment in a batch furnace, named as a rolling and partitioning (R&P) method. By the R&P method, the microstructure of steel is controlled to the mixture of a strain-induced martensite as the matrix phase, and an optimum amount of retained austenite as the second phase which is dispersed in isolation and surrounded by the transformed martensite.
In this paper, the microstructure formation during the R&P process and the deformation mechanism that would bring about the excellent strength–ductility balance are discussed based on the results obtained from the in situ neutron diffraction measurement. The results have revealed that the typical Lüders-like stress–strain curve of R&P steel is caused by competitive plastic flow between austenite and martensite, and an effective transformation induced plasticity phenomenon.
Comparison of the YS-El balance of mass-produced conventional steels and newly developed steels by the R&P method.24) (Online version in color.)
A rapid carbonization process of biomass to indirectly utilize exhaust heat by using heat storage materials (HSM) is proposed. In this process, carbonization proceeds by the heat transferred from HSM balls and the simultaneous pulverization through collisions with balls. This paper deals with the synergistic effect of the pulverization of biomass on its carbonization behavior. Biomass samples were charged together with stainless balls into an electrically heated rotary kiln-type reactor. Carbonization and pulverization experiments were carried out through both simultaneous and separated processes. The formed chars were characterized through evaluation of carbon crystallinity and observation of morphology.
The linear relationship between yields of char and carbon crystallinity was confirmed. The proposed process promoted both carbonization reaction and pulverization phenomenon. Carbon crystallinity increased with increase of carbonization time and became to be lower than that separated process. Biomass samples started to deform after 1 min in the simultaneous process due to melting of biomass surface. Deformation of char with higher stress resulted in higher pulverizing ratio and also it seemed to accelerate the pyrolysis/volatilization of organic compounds such as lignin.
In this study, supply chain patterns of steel products are investigated from the viewpoints of quality assurance responsibility and understanding of physical phenomena in steel. This study focuses on the differences in supply chain patterns between steel nails for common use and valve springs for the automotive industry. In the supply chain of steel nails for common use, which takes a conventional pattern from raw materials to final products, the quality of each supplier’s product is guaranteed just by the Japanese Industrial Standards (JIS), and no supplier takes quality assurance responsibilities beyond its business range. By contrast, in the supply chain of valve springs for automotive use, each supplier takes quality assurance responsibilities for the final product beyond its business range, and the suppliers cooperate with one another to fulfill stringent quality requirements by automotive manufacturers. Therefore, the supply chain pattern of valve springs is different from the conventional pattern of common use steel products like steel nails. It was also found that the supply chain pattern of valve springs can be caused by the insufficient understanding of physical phenomena in steel, martensitic transformation and hardening in this case. This study suggests that the conditions that determine the supply chain pattern of a steel product could be business practices for quality assurance, namely based on standard specifications or users’ requirements, and the natural scientific understanding level about physical phenomena in steel. Although this study focuses on steel nails and valve springs, this finding is applicable to other steel products.
This study aims to visualize an instantaneous event of intermittent splashing from a bath surface caused by gas blowing from a top lance. The visualization techniques include computational fluid dynamics (CFD) and an experimental procedure using a high-speed camera. A remarkable finding is the fact that the difference between the air pressure inside the cavity and hydrostatic pressure outside the cavity forms a constriction and causes a strong intermittent splash.