The bed permeability state prediction model of sintering process based on data mining technology was proposed in this study. Firstly, the sintering production data were analyzed by fuzzy clustering algorithm, to make a comprehensive evaluation of the bed permeability state. Then the prediction model of bed permeability state was established via support vector machine, based on the sample data that obtained in the cluster analysis. The bed permeability prediction model has a good learning and generalization ability, its prediction hit rate reached 87.5%. The practical application showed that: the sintering process could be stabilize effectively, since the operation parameters was adjusted according to the prediction results of bed permeability state; the standard deviation of burn through temperature and burn through point was decreased by 47% and 34% respectively.
CF is regarded as the best bonding phase with superior strength and reducibility in sintering. The comparison of hematite and CF reduction process was made in this study. Isothermal reduction experiments of powdered hematite and CF in a continuous stream of 30% CO and 70% N2 at 1123 K, 1173 K and 1223 K were conducted through thermo-gravimetric analysis (TGA). Reduction rate analysis revealed that the reduction of hematite comprises three independent steps (H→M→W→I), whereas that of CF mainly comprises two steps (H→M→I). The reduction of powdered hematite and CF can be described by shrinking layer model, not shrinking core model completely. Results of the ln-ln and Sharp analysis methods indicated that the reduction of hematite included two kinetics stages, namely, plane-like mechanism and then spherulitic-type mechanism. The reduction of CF only included the plane-like mechanism stage. The model free method revealed that the activation energy of hematite and CF reduction were 5.81 kJ·mol−1 and 46.89 kJ·mol−1, respectively.
Desulphurization has been of more importance to control the quality of hot metal in blast furnace (BF) with increasing utilization of low-grade iron ores and poor-quality fuels as well as reducing the slag volume by decreasing MgO concentration. To improve performance with better understanding the desulphurization in BFs, sulphide capacities of CaO–SiO2–Al2O3–MgO system relevant to BF slags were experimentally determined using gas-slag equilibration technique. The results show that sulphide capacity increases with the increase in temperature, CaO/SiO2 weight ratio and MgO at fixed Al2O3 concentration. The experimental results were further compared with sulphide capacity obtained from available models in open literature. In addition, the equilibrium sulphur partition ratio between slag and hot metal was calculated from sulphide capacity with oxygen activity in the melt, which is also compared with industrial data and the laboratory experiments of kinetic desulphurization of hot metal.
This study investigates the dominant factors affecting the strength of ferro-coke, which is produced by blending iron oxide with coal particles, with the addition of hyper-coal (HPC), to produce a high reactivity and strong coke. A diametral compression test for ferro-coke with and without HPC addition is performed. A three-dimensional ferro-coke model is then developed using micro X-ray computed tomography, and the relative proportions of pore, pore wall, iron, and pore space surrounding the iron particles, termed here “defect”, are quantified using this model. Moreover, a stress analysis is performed for the ferro-coke model. The diametral compression tests indicate that the strength of ferro-coke increases with the increasing blending ratio of HPC. The image-based modeling indicates that the wall thickness increases and stress concentration is relaxed with increasing addition of HPC due to enhancement of the adhesiveness of coal particles. On the other hand, the relative proportion of the “defect” is independent of HPC addition. Therefore, ferro-coke strength is found to be determined not by the “defect” around iron oxide but by the wall thickness.
Average maximum principal stress distribution in sample D.
Ilmenite concentrate is a superior quality raw material to production of titania slag. The iron oxide of ilmenite concentrate is reduced to metallic iron and titanium oxide enriched in slag. Therefore, the enhancement reduction of ilmenite concentrate is in favor of improving the level of smelting of titania slag and reducing smelting time and decreasing energy consumption. The effect of Na2B4O7 on the carbothermic reduction of ilmenite concentrate was investigated, and the addition levels were selected as 0%, 1%, 2%, 3% and 4%, respectively. It is concluded that, under the constant reduction conditions, the metallization and particle size of iron of reduced ilmenite concentrate showed an increase first and then a slightly decrease with increasing the content of Na2B4O7, while FeO and residual carbon showed an opposite tendency. Maximum metallization rate and the largest size of metallic iron were 89.5% and 113 µm at 2% Na2B4O7 content, respectively. FeO content was decreased from 11.58% to 4.23% and then increased to 5.81%, while residual C from 2.73% to 2.34% and then to 4.2%. The main phases of all the reduced samples were (Fe Mg)xTiyO5, Fe and TiO2. The TG and DTG curves indicated that the mass loss of ilmenite concentrate with Na2B4O7 addition was obviously more than that without Na2B4O7. Na2B4O7 can reduce the reduction temperature of ilmenite concentrate, and the extent of the temperature reduction was 80, 114, 111 and 119°C, respectively.
Carbothermic reduction of chromite is an important industrial process for extracting chromium from the chromite. To have a better understanding of the effect of iron on the carbothermic reduction of chromite, the reduction of synthetic chromite (FeCr2O4) by graphite with/without the addition of iron powder was investigated in this paper by Thermogravimetric Analysis (TGA) in argon atmosphere. The fractional reduced samples were examined by SEM/EDS and XRD analysis, and the reduction process was thermodynamically and kinetically evaluated. The experimental results show that the iron powder addition enhances the reduction of FeCr2O4 and this effect increases when increased amounts of iron powder are added. This phenomenon is attributed to the in situ dissolution of chromium into the iron and mixed carbide (Cr,Fe)7C3, which can decrease the activity of the nascent chromium formed by the reduction of the FeCr2O4. The experimental results indicate that the reduction of FeCr2O4 with up to 80 wt.% iron powder addition is likely to be a single-step process and the kinetic analysis suggests that the reduction reaction is likely to be either (a) chemical reaction at the surface of FeCr2O4 or (b) diffusional dissolution of the product (FeCr2) into the iron/alloy particles or the mixed control of (a) and (b).
The effects of minor elements TiO2, MnO, Na2O, K2O, CaS, and B2O3 to the liquidus temperatures of blast furnace slags were experimentally studied. The base blast furnace slag composition in SiO2–Al2O3–CaO–MgO system is fixed at CaO/SiO2=1.1, 16 wt% Al2O3 and 8 wt% MgO. The liquidus temperatures of the synthetic slags have been determined by high temperature equilibration, quenching and Electron Probe X-ray Microanalysis (EPMA) techniques. It was found that within the present investigated composition ranges, the additions of minor elements to the BF slags remain in melilite primary phase field. The minor element decrease the liquidus temperatures of the blast furnace slags in different extents and follows the tendency: B2O3 > CaS > Na2O > K2O > “TiO2” > MnO. The present measurements were also compared with FactSage predictions.
Gas stirring plays a significant role in steelmaking process. The stirring effect is often assessed by the mixing time. In the past, the effects of many factors such as the number and position and relative angle of porous plugs in a ladle, as well as gas flowrate on the mixing time have been studied and some beneficial results used in industrial practice. However, for a ladle with dual plugs, the researches on gas flowrate basically focused on the blowing mode with the same gas flowrates for every plug (Mode-S), while the mode with different flowrates (Mode-D) has not yet been reported. In the present work, a water model for a 120 t ladle is carried out to mainly compare the effect of the two gas blowing modes on mixing. Two porous plugs are located at 0.55–0.70R (R is the radius of ladle bottom), with different relative angles (45–180°) and total flowrates (6.92–18.45 NL/min). The results show that Mode-D can significantly change the mixing time. Compared with Mode-S, the mixing time is respectively decreased by 50 seconds at 6.92 NL/min and 30 seconds at 18.45 NL/min when the plug positions are located at 0.64R. The results of mathematical simulation explain the phenomena. In the Mode-D, the strong gas plume forms a larger circulation flow to stir the ladle and the weak plume forms a smaller one. Under this case, the interference and collision from two plumes are weakened and the dissipation of stirring energy is decreased, thus the mixing time is shortened.
The average residence time, metal circulation rates and percentage tap weights of metal suspended in the emulsion of a Basic Oxygen Furnace (BOF) are calculated under defined assumptions. Data has been taken from the IMPHOS (Improved Phosphorus Refining) project in which a 6 tonne BOF was simultaneously sampled at selected depths and at selected times under controlled operating conditions.1) A discussion is given on the formation of the model and as to the diversion from previously reported findings from this data.2) Links have been made between the macroscopic parameters calculated and the chemical refining performance seen through the heats. X-ray computer tomography has given information on droplet size and numbers in the emulsion. With the data used to validate the proposed model.
The present study aimed at completing the direct alloying operations of Si in liquid steel by the way of electrochemical method. In the traditional steelmaking process, the ferrosilicon is prepared by preliminary smelting the oxide ores, and then added into the liquid steel for alloying. In this study, the direct alloying operations will be completed by electrolyzing CaO–SiO2–Al2O3 molten slags, in which process the alloy elements are produced in the slag-steel interface and then diffuse into the liquid steel when direct current pass through the two electrodes which are placed in liquid steel (cathode) and slag (anode), respectively. It is a new attempt to applying the electrochemistry on the steelmaking process. Experimental results showed that after electrolysis for 2 hours, as applying larger current and using slag with a higher content of SiO2, there will be a higher content of Si generated in the liquid steel. It was found that during the electrolysis process, Al content in the steel also slightly increases, while there was no obvious change of Ca content. Meanwhile, reduction of Al2O3 and CaO is not the main factor to decrease current efficiency (calculated by the ratio of generated Si in liquid steel to the theoretical silicon by Faraday law).
To find the effect of double ruler electromagnetic breaking (EMBr) in continuous casting flow control (FC) mold, present investigation aims at developing a validated transient magnetohydrodynamic (MHD) flow and turbulence model for the FC mold caster of the Tata Steel plant. Towards the above, the mathematical model firstly validated with the online plant measurement data and found to be in good agreement. Based on the plant measurement of magnetic fields at various EMBr ruler current settings, simulations are carried out for different casting speeds and section sizes. Heavy meniscus flow instabilities during EMBr OFF conditions are found to be suppressed through the retarding influence of applied magnetic field. The effect of EMBr causes thinning of the velocity boundary layer and the flow is found more prone to be surface directed. In the mold, magnetic field dampens turbulence and emboldens the formation of large scale vertical structures, whose axis is aligning with the magnetic field vector. The jet hitting the narrow wall becomes more focused and reduced lateral dispersion during EMBr ON condition. Change in EMBr ruler positions leading to a shallower downward jet angle and higher surface velocity.
In order to investigate the interfacial reaction phenomena between the continuous casting molten steel with high Mn content and CaO-SiO2 based mold flux, a series of laboratory experiments were carried out. Various factors which may affect the composition evolution in mold flux were taken into account to propose a reaction mechanism. Based on the results obtained experimentally, four types of mold flux representing different stages of composition evolution were used to elucidate lubrication and crystallization behaviors of mold flux during the reaction process. The results show that the chemical reaction, 2[Mn] + (SiO2) = [Si] + 2(MnO), is mainly governed by the Mn content in molten steel. The initial (CaO/SiO2) in mold flux has a moderate effect on this reaction. The couple effect of MnO accumulation and SiO2 reduction induces a remarkable decline in viscosity and break temperature of mold flux. Crystallization temperature shows a downward trend after the first rise. The formation of Ca4Si2O7F2 takes place first in all mold flux samples. The morphology of cuspidine changes from small faceted shape with regular spacing to larger interconnected block, and the number of cuspidine crystals decreases with MnO accumulation and SiO2 reduction. In mold fluxes with higher MnO content, the micro tephroite crystals precipitate at lower temperature, while the crystallization of CaF2 is suppressed, which results in higher heat flux and thinner solid slag film. The findings in this study provide the evidence of heat transfer deterioration in flux layers during the continuous casting process for high Mn steel.
Grain growth in nanometer scale is closely investigated with a combination of a large-scale molecular dynamics (MD) simulation and a comprehensive post-analysis technique. The volume change of grains is directly estimated for all grains in two-dimensional and three dimensional grain growths. For the two-dimensional grain growth, grains with seven and more neighboring grains generally grow larger, whereas those with five and less neighboring grains shrink and some of them disappear within the timescale of the simulation. The result agrees with the von Neumann–Mullins relation. For the three-dimensional grain growth, threshold number of neighboring grains is estimated to be approximately 14, which is close to many of reported values from previous experiments and simulations. An extended model of the von-Neumann-Mullins relation for the three-dimensional grain growth is derived based on the MacPherson-Srolovitz model, from which the threshold number of neighboring grains is estimated to be 14.7. Using the von Neumann–Mullins relation, grain boundary mobility is estimated to be in the order of 10 × 10−9 m4J−1s−1, which is within the range of reported values. Results and discussion derived from the large-scale MD simulation basically agree with the classical theory, which proofs the validity of simulation results from the statistical point of view, whereas most of present MD studies still limits the discussion to the local structure around of particular grain boundaries due to the size limitation. The quantitative discussion based on the large-scale MD simulation is largely attributable to the rapid progress in high-performance computational environments.
This paper proposes a molten steel level measuring method in a tundish. This method is based on the temperature attenuation characteristic of the temperature tube. Our previous work focused on the temperature distribution of the temperature tube which can be used for molten steel level measurement. In some continuous casters, sticky slag adheres to the tube, thus, blocking the temperature information on the tube. To avoid this problem of lacking true temperature, we analyzed the principle of adhesion and conclude that the adhesive slag thickness of the tube is influenced by whether that part of the tube contacts the slag or the molten steel prior to being lifted. Consequently, for detecting the adhesive slag thickness online with available pyrometer, a heat transfer model is established. Through analysis of the model, it was revealed that the thickness of the adhesive slag remarkably influences the measurable maximum curvature time of the temperature attenuation. In addition, this maximum curvature time is insensitive to other irrelevant factors such as initial temperature and ambient temperature. Therefore, the maximum curvature time of the temperature attenuation of the adhesive slag indirectly indicates the molten steel level. Finally, the experiments prove the feasibility and the sound accuracy of this method with the maximal error being 3 mm. This method is not only suited for the molten steel level measurement, but also applicable to other non-destructive measurement of coating layer thickness.
Energy allocation in iron and steel industry is the assignment of available energy to various production users. With the increasing price of energy, a perfect allocation plan should ensure that nothing gets wasted and no shortage. This is challenging because the energy demand is dynamic due to the changes of orders, production environment, technological level, etc. This paper try to realize on-line energy resources allocation under the situation of dynamic production plan and environment based on typical energy consumption process of steel enterprises. Without definite analytical model, it is a tough task to make the energy allocation plan tracks the dynamic change of production environment in real time. This paper proposes to deal with dynamic energy allocation problem by interactive learning with time-varying environment using Approximate Dynamic Programming method. The problem is formulated as a dynamic model with variable right-hand items, which is an updated energy demand obtained by on-line learning. Reinforcement learning method is designed to learn the energy consumption principle from the historical data to predict energy consumption level corresponding to current production environment and the production plan in future horizon. Using the prediction results, on-line energy allocation plan is made and its performance is demonstrated by comparison with static allocation method.
Dysprosium (Dy) is expected to be recovered from end-of-life neodymium-iron-boron (NdFeB) magnets in the near future because of its worldwide shortage. Therefore, the rapid on-site elemental analysis of dysprosium in end-of-life NdFeB magnets is required. Here, we report a method of measuring the dysprosium composition in a NdFeB magnet on-site using a portable total-reflection X-ray fluorescence (TXRF) spectrometer. This method leads to drastic reduction of dysprosium loss caused by dissolving a NdFeB magnet because the portable TXRF spectrometer requires a small sample volume for measurement and its detection limit is as low as the ppb level. A NdFeB magnet was dissolved into hydrochloride acid and then iron in the solution was extracted using 4-methyl-2-pentanone (methyl isobutyl ketone: MIBK). Yttrium oxide (Y2O3) and a diluted standard solution of rubidium (Rb) were added to the solution as internal standards. We measured the X-ray intensities of dried residue of the solution using a portable TXRF spectrometer. Dy Lα line was clearly detected in the solution, whereas it overlapped with Fe Kα line in the solution before the MIBK extraction process. The dysprosium composition in the NdFeB magnet was determined from the measured intensities of Dy Lα, Y Kα, and Rb Kα, the relative sensitivities of dysprosium and yttrium to rubidium for the portable TXRF spectrometer, and the weights of the dissolved NdFeB magnet and yttrium oxide. The calculated dysprosium composition was in good agreement with that obtained by conventional ICP-AES.
X-ray diffractometry (XRD) with Rietveld refinement was applied to the crystalline phase analysis of two certified standard sintered ores (JSS 851-2 and JSS 851-5) and an industrial sintered ore (SO-1). An eskolaite (Cr2O3) powder was mixed at 10 mass% with each sintered ore sample as an internal standard to correct for the effect of amorphous phase and unknown crystalline phases. The results of Rietveld refinement for JSS 851-2 were 23.0 mass% hematite, 29.5 mass% magnetite, 39.8 mass% silico ferrites of calcium and aluminum, 5.6 mass% dicalcium silicate, and 2 mass% amorphous phase, and these values are within the range previously reported for its phase composition. The elemental concentrations calculated from the crystalline compositions using Rietveld refinement of the three sintered ore samples were also in good agreement with the certified values and those from X-ray fluorescence analysis. A calibration curve method and a diffraction–absorption method for crystalline phase quantitation were used to validate the results of the powder XRD/Rietveld method, and the three results agreed well. The present method enables reliable determination of not only the major crystalline phases but also the amorphous phase present in the sintered ore.
This paper suggests a rapid preparation for quantitative analysis of a type of high-alloyed steel in wavelength dispersive X-ray fluorescence analysis of the allying elements (vanadium, chromium, cobalt, molybdenum, and tungsten). The samples were decomposed with nitric acid and hydrofluoric acid, and then fused with an alkali flux of lithium tetraborate. The synthetic standards for the quantification were prepared by using metal standard solutions of analytes and iron as a matrix component. Analytical results based on the following methods were successfully obtained: a milligram-based calibration with six synthetic standards by using conventional linear calibration curves, and a corrected fundamental parameter calculation with the same synthetic standards. The present method enabled rapid sample preparation and thus simplified an analytical procedure for high-speed steel compared to the conventional wet analysis including inductively coupled plasma atomic emission spectrometry.
In the hot rolled plate manufacturing process, thermo-mechanical control process (TMCP) technology is widely used to improve controlled cooling technology. A new cooling system based on inclined staggered impinging jets was developed and adopted in a commercial steel mill. This study mainly focused on the surface flow field and heat transfer coefficient of the new cooling system. The flow field was numerically studied, and a method was developed to calculate the average heat transfer coefficient. And the relationship between jet velocity and average pressure in the impinging area was investigated. The study revealed that the average pressure almost increased linearly with improving the jet velocity. Meanwhile, the average heat transfer coefficient was observed to increase with average pressure in the range of 524–2989 Pa. However, heat transfer coefficient increased by only 8.4% with the plate moving speed of 0.7 m/s when the average pressure increased from 1623 to 2989 Pa. Additionally, the experimental data were useful to predict the cooling rate and microstructure of the steel product.
In hot strip rolling, the work roll shift method has been widely used to disperse thermal crown and wear of the work rolls in the axial direction. This paper provides a strategic control method for the work roll profile which surpasses the conventional work roll shifting method. A numerical simulation model which enables prediction of thermal crown and wear of work rolls with high accuracy has been developed, and a new shifting method has been proposed, focusing on the problems of the conventional shifting method.
In the conventional shifting method, the thermal crown within the contact area is calculated with a fixed cyclic stroke and step. In that case as the stroke increases, the average value of thermal crown in a rolling campaign decreases without concentration of heat input from the strip to the axial center of the work roll. However, as the stroke increases, the standard deviation of thermal crown during the campaign increases.
Although the work roll shift method is effective for dispersing work roll thermal crown and wear, the thickness profile of the strip is affected by the positional relationship between the work roll and the strip. Therefore, for further improvement of the work roll shift method, the need for a flexible shift method which considers the positional relationship between the work rolls throughout the entire rolling campaign is suggested.
Cooling system design is one of the key technologies for the hot stamping process, and the properties and the production efficiency of the advanced high strength hot-stamped parts are greatly influenced by the cooling performance of the hot stamping dies. In this paper, a new selection criterion for the cooling channel parameters including the channel diameter, the number of the channels and the distance from the channel center to the die contact surface was developed, and a cooling channel structure optimization method based on the difficult degree of cooling was proposed. Using the criterion, cooling channels of the hot stamping dies for an anti-collision beam were designed, and the effect of the designed cooling systems was measured according to the cooling intensity, uniformity and the die strength by simulation. Based on the simulation results, the structure of the cooling channels was optimized. Results show that all the cooling systems designed based on the cooling channel design method can meet the requirements of the cooling intensity and the die strength, while cooling systems with smaller cooling channel diameter are recommended to be the better choices in terms of the cooling uniformity. In addition, the cooling uniformity and intensity of the cooling system optimized based on the difficult degree of cooling can be both improved. According to the simulation results, the hot stamping dies for the anti-collision beam were manufactured, and the high quality hot stamped part was achieved, which proved the rationality of the cooling channel design method.
For further improvement of press formability of steel sheets, it is important to clarify the relation between macroscopic mechanical properties and microstructure under multi-axial deformation state. The objective of this work is to correlate the work hardening behavior with microstructure evolution under biaxial tensile state. The materials used in this study were interstitial-free (IF) steels with different grain sizes. First, the work hardening behaviors were examined by conventional uniaxial tensile and bulge tests, and the differential hardening behavior was measured. Next, in-situ observation of microstructure evolution was conducted for uniaxial and biaxial tensile deformation using the microscopic biaxial tensile system with electron back scatter diffraction patterns (EBSD) with scanning electron microscope (SEM) analysis. The texture development and inhomogeneous deformation were observed clearly under the biaxial tensile state. The effect of texture development on the yield locus was investigated using Taylor-Bishop-Hill (TBH) theory and the orientation distribution function (ODF). It was found that the differential hardening at the grain level was caused by the crystal rotation. Finally, the inhomogeneous deformation was analyzed. The material with fine grain exhibited relatively homogeneous deformation and high work hardening ratio even over a large strain range under biaxial tensile state. It was suggested that the texture development over a large strain range and the homogeneity of deformation resulted in the differential hardening behavior.
The effect of Mg3(PO4)2 and SrCrO4 pigments in paint layer on the corrosion behavior of 55 mass% Al–Zn coating at the cut edges and the delamination mechanism of the organic paint at cross-cut areas of pre-painted galvalume steels were examined using wet/dry cycle corrosion tests. The Mg3(PO4)2 pigment did not inhibit white rust formation at the cut edges and the delamination of the organic paint. On the other hand, the SrCrO4 pigment suppressed white rust formation and improved the delamination resistance of the organic paint. Observations of the cross sections of the cross-cut area indicate that delamination was the result of the formation of corrosion products in the Al–Zn layer under the organic paint layer, thus the SrCrO4 pigment inhibited the delamination of the organic paint layer by reducing the corrosion rate of the Al–Zn layer. The Kelvin probe force microscopic measurements revealed that while the addition of the anti-corrosive pigments increased the corrosion potential of the Al–Zn layers, it did not affect the electrochemical properties of the steel substrates. The increases in the corrosion potential can be attributed to the barrier effect of the corrosion products containing chromate pigments on the Al–Zn layer against anodic dissolution. From these results, it is reasonable that chromate pigments effectively increase the delamination resistance of organic paint layers on pre-painted galvalume steels under atmospheric corrosion conditions. In addition, phosphate pigments would improve the corrosion resistance of Al–Zn layers in atmospheric environments with low chloride concentrations.
In this study, diamond-like carbon (DLC) films are prepared by DC-pulsed plasma-enhanced chemical vapor deposition (PECVD) after the oxynitriding treatment of PM60 high-speed steel. The chief study of the parameters of a DC-pulsed PECVD process includes various power densities (200, 400, 600, 800 and 1000 mW·cm−2) with unipolar negative-pulsed voltage. In order to evaluate the properties of the DLC films for DLC/oxynitriding-treated PM60 high-speed steel, Raman spectroscopy analysis, wear tests, hardness tests, Rockwell indentation and corrosion resistance inspections are performed. The experimental results show the duplex coating layers to have the ideal properties when the DLC films are treated by the unipolar negative-pulsed voltage, a deposition time of 90 min and duty cycles maintained at 11%, with an appropriate power density (400 mW·cm−2), respectively. The DLC/oxynitriding duplex treatment results in the highest surface hardness (Hv0.2 2516.4) and lowest wear volume loss (when the load of 2 N and 5 N is 2.36 × 10−3 mm3 and 5.67 × 10−3 mm3, respectively). In addition, when the DLC/oxynitriding duplex film is treated by a power density of 200 mW·cm−2, it possesses the lowest corrosion current (Icorr = 7.24 × 10−5 A·cm−2) and highest polarization resistance (Rp = 6.26 × 102 Ω·cm2) in 3.5 wt% NaCl solutions. The test results confirm that the optimal wear and corrosion resistance can be acquired by the DLC/oxynitriding duplex treatments.
Study on continuous cooling transformation (CCT) behavior is an essential issue before thermo-mechanical processing for a new steel. In this study, dynamic CCT characteristics and microstructural evolution of a novel Cu-bearing pipeline steel with different Cu content (1.06%, 1.46% and 2.00%) were investigated by means of a combined method of dilatometry and metallography. The microstructure developed at a cooling rate range of 0.05 to 30°C/s consisted of pearlite, polygonal ferrite, quasi-polygonal ferrite and acicular ferrite. More Cu addition could lower the transformation temperature for austenite to ferrite and lead to an increase in the driving force for the acicular ferrite transformation, resulting in a full acicular ferrite for 2.0 Cu steel at cooling rate above 2°C/s. The precipitation behavior of Cu-rich phase during continuous cooling showed that Cu precipitation could occur in the acicular ferrite, which made a hardness peak on the hardness vs cooling rate curve of 2.0Cu steel at the cooling rate of 2°C/s. However, no Cu precipitate was detected in the acicular ferrite at higher cooling rate for 2.0 Cu steel. Higher supersaturation of Cu in austenite and a short incubation period of Cu-rich phase precipitation were assumed to allow the Cu precipitation to occur in the auto-aging after acicular ferrite transformation.
We investigated the microstructural evolution during intercritical annealing in Nb-added low-carbon steels, focusing on the synergistic effects of the addition of Nb and the ferrite (α) to austenite (γ) phase transformation on the recrystallization behavior of α. Two kinds of specimens, containing 0.02 and 0.05 mass% Nb, were prepared and annealed at the intercritical temperature (750°C) for a long time. The progress of recovery and α recrystallization was retarded by increasing the amount of Nb addition during intercritical annealing. Moreover, the progress of α recrystallization during intercritical annealing was mainly attributed to continuous recrystallization due to subgrain growth. The fraction of γ that formed during intercritical annealing increased owing to Nb addition, but it did not increase with increasing amount of Nb addition. These results suggest that the progress of recovery and α recrystallization was retarded during intercritical annealing by the addition of Nb, thereby causing the increase in γ fraction. Furthermore, the increase in γ fraction led to further retardation of α recrystallization and α refinement with the addition of Nb.
In order to investigate influence of dislocation density on hydrogen embrittlement behavior, local mechanical properties of pure Fe with different dislocation densities were measured during hydrogen charging at various load duration times by electrochemical nanoindentation. For as-annealed samples and severely plastically deformed ones with low temperature annealing, hydrogen charging did not change nanohardness at any load duration times between 1 s and 10800 s. On the other hand, for cold-rolled samples and severely plastically deformed ones without annealing, hydrogen charging caused softening, and the degree of the softening increased at longer load duration times. Consequently, it was found that hydrogen causes softening for samples with higher dislocation density at slower strain rates. The observed softening seems to be caused by increase in dislocation mobility or suppression of work-hardening due to hydrogen atoms trapped around dislocations.
In order to clarify ductile fracture behavior such as void nucleation, crack initiation and crack propagation in bainite-MA dual phase steels, instrumented Charpy impact tests and static bending tests with Charpy specimens were conducted (here, “bainite” has the same meaning as bainitic ferrite). In bainite-MA steels with smaller MA volume fractions and finer MA sizes, the sample showed higher Charpy absorbed energy with higher crack initiation energy and crack propagation energy. From SEM observation of the instrumented Charpy fracture specimens, it was found that void nucleation was enhanced around the boundaries between bainitic ferrite and coarser MA. Based on the results of the static bending tests with Charpy specimens, bainite-MA steels with smaller MA volume fractions and finer MA sizes showed higher void nucleation strain and crack initiation strain. From a finite element analysis of local plastic strain around the boundaries between bainitic ferrite and MA, it was concluded that the equivalent plastic strain decreases in the case of finer MA and longer distance between two MAs compared to coarser MA and shorter distance between two MAs.
Micro-tensile testing and numerical analysis using a crystal plasticity finite element method (CPFEM) were employed to elucidate the deformation behaviour of bainite/martensite structures of a low-alloy steel. The bainite single-phase specimens exhibited habit-plane-orientation-dependent yielding, similar to the martensite single-phase specimens. In the bainite/martensite dual-phase specimen, deformation concentrated in the bainite region oriented favourably for in-habit-plane slip, leading to low-ductility fracture. With consideration of the habit-plane-orientation-dependent yielding, the present CPFEM analysis successfully reproduced the anisotropic plastic deformation behaviour of the single-phase steels observed in the experiments. The numerical results for the bainite/martensite specimen showed slip localization in the bainite region and stress concentration near the interphase boundary. This suggests that the interphase boundary can be a site for the fracture origin.
Mechanical properties of simplified model of ferrite/cementite lamellar structure in pearlite steel wire are examined by a strain gradient crystal plasticity analysis. Bagaryatsky and Pitsch-Petch relationships are used to determine crystallographic orientations of ferrite and cementite phases. Obtained results show that yield stress and strain hardening rate increase with reduction of lamellar thickness of ferrite layer and this increase is larger in the model with Bagaryatsky relationship than that with Pitsch-Petch one. Detailed mechanism that leads to this significant change in macroscopic mechanical response is discussed from the view point of dislocations’ behavior. Strain incompatibility effect between phases on the mechanical response is shown to be relatively small.