Spring steel wires are extensively utilized in automotive engines valve and suspension applications. Owing to spring often work under high-frequency dynamic loads and periodic alternation stress, non-deformable inclusions often act as fatigue fracture origin of spring steel. The control of inclusions in spring steel for automobile are extensively reviewed. On the one hand, the new perspective on the problems left over the past decades, include the new viewpoint on whether FeO is an inherent constituent of inclusions, the new understanding of the origin of CaO-based inclusions, the new perspective of whether the VD treatment progress should be removed but RH retain, have been discussed. On the other hand, the new approaches, via, calcium treatment, alkali oxide treatment, rare earth treatment, new refractory application, have also been summarized. Finally, the unsolved problems, the source of CaO-based inclusions, the mechanism of alkali metals modified inclusions, the based thermodynamic data for reactions between rare earth yttrium (Y) and non-metallic inclusions, the operability of CaO-containing refractory in industry, that should be explored further are also been discussed.
This paper is concerned for tracking control of displacement control system of continuous casting mold (CCM) driven by servo motor with time-varying load disturbance and parameter perturbation. To improve tracking performance of CCM system, a nonlinear tracking controller based on feedforward and feedback control strategy is presented. Firstly, the system model is established by analyzing the non-sinusoidal vibration device. Then, an extended state observer (ESO) based on fractional power function (FPF) is introduced to estimate the disturbances of the system, which mainly include the parameter uncertainties and time-varying load torque. Subsequently, a composite controller, which includes a feedforward controller based on ESO and a feedback controller based on sliding mode control (SMC), is derived for addressing the tracking issues. Finally, numerical results show that the proposed control strategy is more effective than a feedback controller based on SMC.
Schematic diagram of composite control strategies based on ESO. (Online version in color.)
The effect of silicon content varied from 0 to 45 mass% on the desulfurization in Fe–Si–S, Fe–Si–Cr–S, and Fe–Si–Ni–S melts was investigated at 1873 K (1600°C). It was found that sulfur can be remarkably removed when the silicon content is more than 30 mass%. From the thermodynamic analysis, it was concluded that sulfur may be primarily evaporated as SiS(g), and the activities of silicon and activity coefficient of sulfur in alloys will increase as increasing the silicon content, which makes the evaporation of SiS(g) more feasible and more sulfur removed.
Thermodynamic behavior of nitrogen in Fe–Cr–Ti–Al–Si–N alloy melts was investigated by measuring the nitrogen solubility and solubility product of TiN and AlN by the metal/gas and metal/nitride/gas equilibration techniques at 1823–1873 K, respectively. The nitrogen solubility data measured in Fe–Cr–Ti, Fe–Cr–Si, Fe–Ti–Al and Fe–Cr–Al alloy melts was thermodynamically analyzed to determine the second-order cross-product parameters of Cr–Ti, Cr–Si, Al–Ti and Cr–Al on nitrogen in liquid iron using Wagner’s formalism. By considering the cross-product effect on nitrogen determined in the present study, the effects of the alloying elements on the solubility product of TiN and AlN in the multicomponent Fe–Cr–Ti, Fe–Ti–Al and Fe–Cr–Al alloy melts were successfully reproduced over the wide temperature range.
Reduction behavior of low grade iron ore-coal composite pellets was investigated in the temperature range of 850–1000°C. Effects of the sodium salt addition on the reduction behavior and kinetics were researched. Reduction samples were analyzed by applying X-ray Diffraction (XRD) and scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) to reveal its function mechanism. Results demonstrated that composite pellets without sodium salt showed poor reduction performance at lower temperatures (below 950°C) for most of wustite phase transferred into fayalite rather than iron. Raising reduction temperature can restrain fayalite formation to some extent, but the effect was limited. However, the reduction process of wustite to iron was improved by sodium salt in temperature range of 950–1000°C for its favourable inhibiting effect on formation of fayalite. SEM-EDS analysis of reduced samples revealed that sodium salt also enhanced the growth of newly formed metallic iron particles. The reduction process of composite pellets was diffusion controlled reaction in the temperature range of 850°C–1000°C. The activation energy decreased from 545.06 kJ/mol to 135.77 kJ/mol as 3 wt% of sodium salt was applied.
In the present work, the calculations of the modified operating line for the COREX process, based on the Rist diagram approach, have been performed. The operating line developed by Rist in 1960’s has been modified by various researchers for improvements in the blast furnace. The idea proposed by Rist was further utilized to perform the heat balance along with the net oxygen balance for the other iron making processes. The operating line has been modified thus accounting the generated H2O and CO2 with respect to Fe, in the process of smelting reduction of ore by coal containing volatile and moisture.
Coke oven gas (COG) injection is believed to improve the quality and yield of sinter in iron ore sintering process. A mathematical model is developed to simulate the sintering process with COG injection, particularly focusing on predicting the quality and yield of sinter. The model is validated by comparing the model predictions with sintering pot test data. Numerical simulations are carried out to investigate the influences of location, quantity and covering area of COG injection on the sintering process. The results show that the location of COG injection has a little influence on its effectiveness on sintering, with that the sinter yield decreases slightly with the delay of COG injection while the mean melt quantity index (MQI) and cooling rate (CR) hardly change. The quantity of COG injection has a great influence on sintering. With a typical COG injection area of 40% and injection location of 60 s after ignition, the sinter yield increases by 33.3%, the mean MQI increases by 9.5% and the mean CR decreases by 37.6% when COG injection is 0.5%. The injection area of COG has some interesting influence on its effectiveness on sintering. Under the condition of 0.5% COG injection from 60 s after ignition, with the increase of injection area, the sinter yield increases prominently first and then decrease significantly, attaining its maximum at 40% of injection area. At the same time, the mean MQI and CR attain their extrema also at 40% of injection area.
The strength of sintered ores is an important property affecting to the permeability of the ore layers of the blast furnace. Since the granulated ores have a composite structure with coarse ores coated by fine ores, melting behavior of those ores is complex including a stepwise melting during sintering. Therefore, it is difficult to predict precisely the strength of sintered ores particularly in the case of ore mixtures consisting with many types of ores. Hence, a prediction method for the strength of sintered ores obtained by mixing several types of ores was investigated. By assuming stepwise melt formation process, two different experimental techniques were adopted to individually evaluate the melting behaviors. The strengthening characteristics of fine ores were effectively expressed by the penetrating length. The penetrating length was reflected by the melt fluidity during heating and depended on the quantity of gangue minerals and combined water, as well as the initial grain size of the ores. In contrast, the strength of sintered coarse ores was expressed by the total porosity after melting with limestone. The total porosity depended on the quantity of combined water and types of gangue minerals. As the governing factors in the melt formation of fine ores and coarse ores were different, the quantitative indices of them were estimated individually and re-combined as the sinter strength index to predict the characteristics of the whole ores mixtures. Results of pot tests revealed that the index correlated well with the measured strength of sintered ores.
Schematic diagram showing the sintering process of pseudo-particles. (Online version in color.)
For achieving a better stirring effect, the coherent jet technology has been widely adopted in the metallurgy field; a key feature of this technology is the use of a combustion flame to protect the main oxygen jet. In this paper, a shrouding nozzle with a Laval nozzle structure using preheating technology is introduced. The effect of the shrouding gas flow rate on the behavior of the main oxygen jet is investigated at room and high ambient temperatures. A computational fluid dynamics model has been built to investigate the flow field of the coherent jet in simulation studies. In addition, an experimental study has been carried out to verify the results of the numerical simulation. Based on the results, the new method improves the shrouding gas velocity and forms a low-density zone, which makes its velocity potential core length 178% and 174% longer than that generated by the traditional method at room and high ambient temperature, respectively. However, the shrouding jet forms a shock wave at the exit of the Laval nozzle, which results in removing kinetic energy from the main oxygen jet. As a result, the axial velocity of the coherent jet is smaller than that of the conventional jet, and the velocity variation increases as the flow rate increases.
In the steelmaking process, MgAl2O4 spinel inclusions diminish steel qualities and cause nozzle clogging based on the high melting point and low deformation of MgAl2O4. Typically, MgAl2O4 spinel inclusions are generated from Al2O3 inclusions with increasing MgO content, meaning ladle treatment does not represent an equilibrium state. However, complex reactions simultaneously occur between molten steel, slag, inclusions, refractory, and alloying elements during ladle treatment. Therefore, it is necessary to develop a kinetic model to predict compositional changes in molten steel, slag, and the inclusions during ladle treatment. Such a kinetic model must be able to simulate the generation of MgAl2O4 spinel inclusions from Al2O3 to control such inclusions. Additionally, MgAl2O4 spinel inclusions can evolve into MgO-rich inclusions with gradually increasing MgO content in Mg–Al-based inclusions. In this study, we developed an enhanced kinetic model based on a coupled reaction model with the compositional changes in Mg–Al-based inclusions. We also investigated the influence of the CaO/SiO2 ratio in slag on the generation of spinel inclusions under industrial conditions for a 210-ton steel sample.
In the present study, the effect of different adding time of Ce–Mg–Fe alloy during the refining process was investigated by laboratory experiments and equilibrium thermodynamic calculation. In experiments, samples from different stages were analyzed by SEM-EDS and EPMA for revealing the evolution mechanism of inclusions. Combined with thermodynamic calculation, results indicated that different adding time can affect the yield of Ce and Mg in steel directly, and the difference of types of inclusions in as-cast samples was caused. CaO-containing inclusions as a main type of inclusions in the refining process can be modified by Ce after adding Ce–Mg–Fe alloy. A high Ce content can promote the formation of Ce–S and Ce–O–S in steel and suppress the precipitation of MnS. In consideration of microalloying effect, Ce–Mg–Fe alloy should be added at a later stage of steelmaking process for ensuring a certain content of Ce and Mg.
A liquid metal flow suppression function of a static magnetic field has been theoretically and experimentally investigated, and this function has been utilized as an electromagnetic brake in a continuous casting process of steel. Because macro segregation is induced by macro scale flow, this function must be a powerful tool for the suppression of the macro segregation. However, effect of the static magnetic field on the flow in a liquid-solid coexisting region has not been clarified until now. In this study, for clarification of the magnetic field effect on the suppression of the liquid metal flow in the liquid-solid coexisting region, a model experiment using a packed bed composed of a liquid tin and solid particles made of copper or alumina has been done. The Reynolds number decreased and the friction factor increased by imposing the static magnetic field in the experiment. This was remarkable when the solid particles in the packed bed were the copper rather than the alumina. Furthermore, an equation which can predict the relation between the friction factor and the Reynolds number in the liquid-solid coexisting region under the static magnetic field imposition has been derived based on the Ergun equation and the Hartmann flow analysis under the condition that the solid particles were perfectly conducting material or insulating material.
The unique in-situ SEM/EBSD observation system, which is available to carry out orientation mapping and microstructure observation at high temperature, was developed to observe ferrite/austenite and martensitic transformation of carbon steel directly. The system has the heating capacity over 1000°C and rapid cooling ability of the cooling rate faster than 100°C/s. The SIM imaging shows very good performance to observe quick change in crystalline grain’s shape, and microstructural evolution of secondary recrystallization grains was successfully observed. Furthermore, the in-situ observation system was applied to observe martensitic transformation of plain carbon steel and nickel contained carbon steel. Crystalline grain shape and its orientation before and after martensitic transformation of the steel are observed at the same sample position. And direct comparison of the microstructures between austenite and martensite is successfully realized.
This study aims at investigating the effect of cooling rate on transformation plastic strain in 0.45 mass% carbon steel. It has been revealed that slow cooling rate increases the magnitude of transformation plastic strain especially at the cooling rate less than 10°C/s. Based on Greenwood-Johnson mechanism (strain accommodation mechanism), transformation plasticity modelling is made using crystal plasticity fast Fourier transform numerical method. It is shown that there are three inter-related mechanisms that control the amount of transformation plasticity as a function of cooling rate: the dependence of yield stress of weaker phase with temperature, the dependence of volume change to cooling rate (higher volume change at higher cooling rate) and the influence of viscoplasticity behaviour that enhances creep strain at high temperature. The model predicts the transformation plastic strain relatively close to those of experimental ones at high cooling rates (typically 10°C/s and above) where phase transformation happens at low temperatures, while it under estimates the transformation plastic strain at slow cooling rates where phase transformation happens at high temperatures. These results show that other mechanisms like creep/viscoplasticity should be considered by the model to predict influence of cooling rate on transformation induced plasticity.
The influence of the Si content of Mn-added steel sheets on the reduction behavior of Fe oxide during oxidation followed by annealing in a 10% H2 atmosphere simulating the Fe oxidation-reduction process was investigated using 5.2 mass% Mn steel with 0, 0.2 and 0.5 mass% added Si. Reduction of the Fe oxide was more rapid in the 0.2% and 0.5% mass% Si steels than in the 0 mass% Si steel. Because Si addition increased the proportion of hematite (Fe2O3) and decreased that of wustite (FeO) in the Fe oxide, the difference in reduction speed due to Si addition is thought to depend on the composition of the Fe oxide. It was suggested that reduction from hematite to magnetite (Fe3O4) induces larger lattice defects in the Fe oxide, resulting in a higher diffusion rate of O.
The effect of hydrogen sulfide (H2S) on the behavior of hydrogen entry into low alloy steel was investigated using electrochemical hydrogen permeation technique. In this study, the hydrogen entry side was galvanostatically charged to control the rate of hydrogen evolution reaction.
At pH of 3.0 in acetic buffer solution with 0.1 MPa H2S environment or that with deaerated environment, the potential, charging current density, and permeation current density were measured.
Hydrogen permeation current density had a linear relation to the square root of hydrogen charging current density, indicating that the hydrogen evolution reaction proceeds under Volmer-Tafel mechanism.
For analyzing the results of this study, the efficiency of hydrogen entry was calculated from the relationships among hydrogen charging current density, hydrogen permeation current density and hydrogen overpotential. It was found that the efficiency of hydrogen entry was drastically higher in H2S environment than that in the deaerated environment.
However, the coverage of hydrogen atoms adsorbed on hydrogen entry side did not change in H2S environment as compared with that in the deaerated environment.
All these results suggested that, although hydrogen coverage was not changed, hydrogen entry was accelerated in H2S environment.
This study elucidates the solubility product of Ti4C2S2 in steels by means of first-principles calculations and thermodynamic analysis. For this purpose, the Gibbs formation energy of Ti4C2S2 was calculated theoretically by considering the effect of lattice vibration and thermal expansion. In addition, the Gibbs energies of the bcc and fcc phases in the Fe–Ti–C ternary system were also obtained using the cluster expansion and cluster variation method. Although some experimental data were considered as required, those results were evaluated as the calculation of phase diagrams (CALPHAD)-type thermodynamic parameters through fitting to the sublattice model. By using those thermodynamic functions, an approximate expression of the solubility product for Ti4C2S2 was derived. The result agrees with an experimental result measured in a relatively large temperature range. Furthermore, the formation behavior of precipitates in typical interstitial-free steels was discussed, incorporating an earlier thermodynamic analysis on the Fe–Ti–S ternary system. The results show that NiAs-type TiS was the main precipitate at higher temperatures and that Ti4C2S2 was the main precipitate at lower temperatures.
High-temperature tensile tests, a cast-slab microstructure investigation, and an in-situ observation of the dissolution behavior of 17%Cr-14%Ni-4%Si-Nb steel during heat treatment were conducted. The ductility suddenly decreased at above 1200°C. A microstructure observation near the cast-slab surface suggests that the solidification mode was a divorced eutectic ferrite-austenite; in addition, the precipitation of Fe16Nb6Si7 (G phase) occurred inside the δ phase and at the δ/γ interface. The temperature at which solidification was completed, as calculated using DICTRA, was 1331°C; hence, the embrittlement at about 1200°C, observed during the high-temperature tensile tests, was different from the I-zone embrittlement caused by the residual liquid phase during solidification. The in-situ observations showed that liquefaction occurred from the δ phase near the δ/γ interface at above 1180°C. The high-temperature embrittlement was attributed to compositional liquefaction, in which the G phase was precipitated inside the δ phase and at the δ/γ interface during the cooling process after solidification provided Si and Ni owing to the dissolution upon re-heating.
Ultrafine ferrite + austenite steels with the chemical composition of 0.1%C-2%Si-5wt%Mn show excellent strength (TS=1200 MPa) and high ductility (TEl=25%) balance, compared to conventional TRIP steels. This steel is expected as the third generation advanced high-tensile strength steels (AHSS). This steel can be produced by a simple intercritical annealing, however, longer annealing time is necessary to obtain appropriate ferrite + austenite structure. It is difficult to produce this steel by continuous annealing process. If the annealing time can be drastically reduced, this new TRIP steels can be commercialized. We focused on the effect of the prior microstructures before annealing on the formation of ferrite + austenite structure. The effect of the prior structure is not clear. Therefore, in this study, two kind of prior structures, ultrafine grained ferrite + cementite and martensite were used in 0.1%C-2%Si-5wt%Mn steels. It was found that the prior structure of ferrite + cementite can form large amount (20%) of austenite in a very short time (600 s). This is because cementite finely dispersed in the structure effectively acts as a preferential nucleation site of reverse transformed austenite and C and Mn are concentrated in cementite to enable a short time formation of austenite. Excellent strength-ductility balance (32000 MPa%) which is superior to conventional TRIP steels is also obtained.
Nominal stress - nominal strain curves and change in austenite volume fraction with tensile strain in 0.1%C-2%Si-5Mn steels and a conventional TRIP steel by In-situ XRD using synchrotron radiation in SPring-8. Dotted lines represent volume fraction of austenite and solid lines represent nominal stress. (Online version in color.)
We have fabricated cylindrical single-crystal micropillars with different diameter (d) ranges of 2–3 µm and 5–6 µm on a specific grain in the 18Cr ferritic stainless steel with a ferrite single-phase microstructure. The initial strain rate at the onset of plastic deformation was controlled by variable loading rate in the used nanoindenter. The strain rate sensitivity of the stress required for the slip initiation were examined using the fabricated micropillars. The present compression tests addressed the shear stress on an activated single slip system of micron-scale single-crystals. Smaller-sized micropillars (d = 2–3 µm) often exhibit intermittent strain bursts. The stress for slip initiation (after an elastic loading) changes depending on the initial strain rate, resulting in a high strain rate sensitivity (m) of 0.12. Larger-sized micropillars (d = 5–6 µm) show a continuous yielding. A slight change in their yield stress depending on the initial strain rate provides a relatively low m of 0.04. It is similar to one of the millimeter-sized specimens measured by conventional tensile tests. These results provide new insights to optimize the specimen size for the micropillar compression test applied for the 18Cr ferritic stainless steels.
(a) Optical micrograph and (b, c) SEM images showing microstructure of the studied steel annealed at 1573 K for 1.8 ks, (d) orientation color map showing the orientation along the normal direction (according the color code of unit triangle) and (e) a representative fabricated single-crystal micropillar.
This study aims to clarify the effect of selenium (Se) on the machinability of AISI 4140 steel in the as-hot rolled (HR) and quenched and tempered (QT) conditions. Machining tests were conducted to examine the progressive tool flank wear and the chip formation. The characteristics of the workpiece materials such as hardness, microstructure, and non-metallic inclusions were investigated. The worn tools were analyzed to characterize the possible deposit formed on the tool surface. In addition, micro-computed tomography (µ-CT) and high-resolution transmission electron microscopy (TEM) were utilized to study the Mn(S,Se) inclusions in severely deformed chips.
The investigation showed that Se micro-alloying did not improve the machinability of the HR 4140 steel. However, in the QT condition, the superior machinability was observed for the Se-treated 4140 steel when compared with the untreated steel. Therefore, the effect of Se on improving steel machinability was considered to be strongly dependent on the structure and properties of metal matrix. The factors influencing the improvement in machinability of Se-treated QT steel are discussed.
The present study aims to elucidate the effect of Cu alloying on the strain capacity of Cu-bearing pipeline steels. The main emphasis was placed on understanding the effects of Cu content (1.0Cu, 1.5Cu and 2.0Cu) and the existence form of Cu (as-rolled and as-aged steels) on the yield stress/tensile stress ratio (yield ratio), uniform elongation and strain hardening exponent. Experimental results show that the engineering stress-strain curves present continuous yielding behavior for the as-rolled steels but discontinuous yielding for the as-aged steels. For both as-rolled and as-aged steels, increasing Cu content increases the yield ratio with an accompanying decrease of uniform elongation. It was found that the as-rolled 1.0Cu steel has one strain hardening exponent (n value), low yield ratio (0.68), high n value (0.18) and high uniform elongation (17.7%), showing an excellent deformation ability. There are two n values for the other steels (1.0Cu as-aged, 1.5Cu and 2.0Cu as-rolled and as-aged), and their n values increase at low stress but decrease at high stress with increase of Cu content. In contrast to the as-aged steels, the as-rolled steels show better strain capacity.
In the present study, the prediction of fatigue life by representing characteristic variations of defects with probability distribution functions was conducted by dividing the fatigue process into the crack initiation and crack propagation. Voids, hard inclusions (Al2O3) and soft inclusions (MnS) in steels were supposed as defects and two prediction models were proposed. Only the life of crack propagation was predicted by Paris law in initial defects model (model A) while the life of crack initiation as well as propagation was predicted by Tanaka-Mura model in crack initiation model (model B). The stress intensity factor using (projected square root area of defects) proposed by Murakami et al. was applied to Paris law in both models. The stress concentration due to defects and Taylor factor were applied to Tanaka-Mura model in the model B. These models were applied to four types of steels and the fatigue life was compared with the experimental results. In case of ductile cast iron including voids, the fatigue life predicted by both models was within the range of the experimental scattering. Although the fatigue life predicted by the model A was not consistent with the experimental results under high and low stress levels in case of Cr–Mo steel including MnS inclusions, the fatigue life predicted by the model B mostly showed a good agreement with experimental results. Therefore, it was demonstrated that the fatigue life prediction considering crack initiation showed higher precision than the prediction without crack initiation.
The effect of magnetic fields on solid-liquid reactions in the Fe–Ga binary system was investigated using Fe/Ga diffusion couples. The reaction and phase growth proceeded by diffusion-controlled process for both 0 and 10 T. It is found that the growth of the intermetallic phases was suppressed by magnetic fields of 10 T regardless of whether α-Fe was in a ferromagnetic or paramagnetic state. The pre-exponential factors in the parabolic coefficient of the Fe3Ga + eutectic region at 0 and 10 T were 2.50 × 104 m2/s and 1.32 × 103 m2/s, respectively. Meanwhile, activation energies at 0 and 10 T were 336 kJ/mol and 318 kJ/mol, respectively, which indicated the ineffectiveness of magnetic fields. That is, reduction of pre-exponential factor of the parabolic coefficient leads the magnetic-field-induced suppression of the reaction in Fe/Ga.
BSE images of Fe/Ga interface annealed at 1023 K for 24 h in a zero field (a) and at 10 T (b).
This paper suggests an improved method for the sample preparation to quantify vanadium in high-speed steel by using flame atomic absorption spectrometry. The suggested method prepared a solidified glass body fused with lithium tetraborate after the sample was decomposed with an acid mixture of hydrofluoric and nitric acids. The obtained borate glass was easily dissolved with nitric acid without any residues. Another novelty of this paper was a simultaneous measurement of a vanadium absorption line and a nickel absorption line in the close wavelength, when a multi-wavelength high-resolution spectrometer system was employed. The nickel line worked as an internal standard and could contribute to more precise quantification of vanadium in high-speed steel samples.