A nitrogen removal experiment was carried out under reduced pressure using a 600-kg-scale induction furnace. Using normal and 17 mass%Cr steel, two methods to promote the reaction were selected, i.e., expansion of the reaction area with CO boiling and reduction of surface active elements, with the aim of promoting the chemical reaction at the metal–gas interface. Kinetic results indicate that surface active elements act on the whole experimental condition, and the rate determination step is predicted to be a chemical reaction at the gas–metal interface. Applying the chemical reaction equation obtained by previous research on high-chromium steel, the nitrogen removal rate can be explained using the same estimation method. The results obtained in the present study indicate the importance of development of an effective method to increase the gas–metal surface area.
Krupp–Renn process is an energy-saving technology for ferronickel production from saprolitic laterite ores, in which a semi-fused zone (1300–1350°C) in the rotary kiln is required for the coalescence of ferronickel particles. Fluxes are crucial for the liquid phase formation during the coalescence stage. In this study, effect of quaternary basicity ((CaO+MgO)/(Al2O3+SiO2) mass ratio) on the melting characteristics of laterite ore blended with various amount of CaO in 100% CO atmosphere are investigated, and the growth of ferronickel particles is also observed. It is shown that the characteristic fusion temperatures first decline and then increase with increasing of basicity, which is closely related with the presenting silicate minerals in the reduced products involving olivine, diopside, monticellite, akermanite and merwinite. At the basicity of 0.8–1.2, the characteristic fusion temperatures drop subsequently before reaching a minimum (~1300°C), due to the generation of low-melting-point diopside. The growth of ferronickel particles is improved by the presence of liquid phase.
Inclusion removal is key in the production of high quality steel. The inclusions are primarily removed from liquid steel by reacting with a liquid slag phase. For efficient inclusion removal, the inclusions transfer across the steel-slag interface to dissolve in the slag. This transfer process is strongly influenced by interfacial phenomena. In this study, the dynamic wetting (θ) of a range of slags in the CaO–Al2O3–SiO2–(MgO) system on solid oxides representing inclusion phases (Al2O3, MgAl2O4 and CaO.Al2O3) at 1773 K was investigated using a sessile drop technique. It was found that for all systems studied θ versus time showed a rapid decrease in wetting in the first 10 s tending to a plateau value at extended times. Further, for basic type ladle slags the plateau value was independent of slag composition and for acid type tundish slags the plateau value decreased with increasing basicity. Through work of adhesion analysis it was shown that ladle type slags appeared more suitable for inclusion removal and that from a wetting perspective calcium aluminate would be easier to remove than spinel and alumina. Choi and Lee’s dynamic wetting model was evaluated and found to not only represent the data well but have physical relevance for the basic, but not the acid, slags investigated.
In order to take the advantage of the large reaction surface of fine iron ore concentrates and expect a high reaction rate without sticking or agglomeration problems, a suspension gas-solid reaction system was designed to explore the feasibility of fast direct reduction of fine iron ore. In this study, upward gas flow was used to prolong the particles’ falling time. Pure silica particles were chosen as the dispersion agent. The Stokes gas-particle model with the relaxation time concept was applied to accurately model the falling process. Highly metallized porous iron particles over 90% of Rd (reduction degree) were obtained at 1273 K with 20.1 s of hydrogen reduction. The morphology evolution characteristics of the fine particles during reduction were investigated via SEM, and a conceptual diagram was formulated in this paper in order to well understand the relationship between the reduction condition and the structural evolution. The shrinking core model was introduced to analyzing the reduction kinetics in this experiment system, which indicates that the microstructure evolution of the particle during reduction can be influenced by temperature and the resistance of internal mass transfer cannot be ignored under this experiment condition especially in the later stage of reduction.
A process with acid leaching followed by hydrogen-based fluidized reduction and melt separation is presented for recovering DRI (direct reduced iron) from high-phosphorus oolitic hematite in this study, and the aim of this study is to provide theoretical and technical basis for economical and rational use of high phosphorus oolitic iron ores. The reducibility of the ore can be improved by acid leaching, which is caused by the formation of voids in the ore particles after acid leaching and enhancing the internal gas diffusion. The phosphorus content in the DRI is still relative high even though there is no carbon in DRI, and it can be decreased to 0.087 wt% (raw ore 1.2 wt%) with the optimum condition in this study. It is proved that P exists in the DRI recovered from melt separation in the form of P2O5 inclusions or FexP as solid solutions, while not in the form of Ca3(PO4)2 inclusions. Finally, a combined flowsheet for the treatment of high phosphorus oolitic iron ore is designed in this study.
Various types of brick masonry are used in the handling and treatment of hot materials in steelmaking processes. Although trouble related to damage of the bricks and joints is an important issue, it is difficult to predict and prevent damage of brick masonry because masonry is characterized by heterogeneity, nonlinearity and nonequilibrium. In this paper, the brick masonry in a coke oven is studied as an example, and a method of deformation prediction under external force is developed as a basic study. First, a compression test of a sample consisting of two bricks with one mortar joint is performed. Second, a part of a heating flue of a coke oven made of bricks is constructed as a test sample, and brick masonry deformation under external force is measured experimentally. Third, a model of the same heating flue in the experiment is prepared, and a numerical elasto-plastic analysis is carried out using the total strain rotating crack model based on the measured mechanical properties. Finally, the experimental and numerical deformation results are compared. The results validated this method of predicting masonry deformation.
Understanding the degradation of blast furnace hearth refractories by coke ash products is a crucial component in achieving long furnace life. This study has been conducted to determine the reactivity of calcium aluminates (CaO·Al2O3, CaO·2Al2O3 and CaO·6Al2O3) in contact with an alumina-carbon refractory at representative hearth temperatures of 1450° to 1550°C. The rate of reaction between the alumina-carbon refractory and calcium aluminates was observed to increase with CaO content. The largest reaction layer was observed in CaO·Al2O3 followed by CaO·2Al2O3 and CaO·6Al2O3. The calcium aluminate (CaO·Al2O3, CaO·2Al2O3) and alumina-carbon reactions observed in this study were found to be consistent with the logarithmic rate law, with the exception of CaO·6Al2O3 where no reaction was observed. Energy dispersive spectroscopy analysis indicated the formation of CaO·2Al2O3, CaO·6Al2O3, corundum (Al2O3), plagioclase (CaO·Al2O3·2SiO2) and melilite (2CaO·Al2O3·SiO2) at the reaction interface of the reaction couples. The large volume changes due to the formation of these phases may result in spalling at the hot face of the hearth refractory.
The influence of different reactivity coke on reduction, softening, melting and dropping properties of iron-bearing burden were studied by melting and dropping experiment, meanwhile reduction mechanism and softening-melting mechanism of stock column, as well as stock column pressure difference were also analyzed. The results show that reduction degree at 1000°C increases, softening-melting start temperature and dropping temperature decrease, and melting temperature rises, when the coke reactivity increases from 24.76% to 56.28%. Softening and melting temperature interval increases, melting and dropping temperature interval reduces and stock column permeability is improved as the coke reactivity increased, which lead to softening-melting and dropping properties value of iron-bearing burden reducing. In addition, industrial experimental analysis shows that reducing agent rate (RAR) can be reduced by 8.26 kg/t using high reactivity coke (HRC) of 12%. Therefore, the appropriate increase of coke reactivity could improve stock column permeability and high temperature properties of iron-bearing burden, so as to reduce RAR.
The importance of energy saving in the ironmaking process is widely recognized. Many energy saving efforts related to ironmaking have already been carried out, and further energy savings by conventional methods are hardly to be expected. The oxygen blast furnace is considered to be a promising process in terms of flexibility of energy use and advantages related to CO2 mitigation. Focusing on energy saving, in this study, the optimum configuration of the ironmaking process based on the oxygen blast furnace was investigated by numerical approaches and case studies. First, because productivity can be greatly improved in the oxygen blast furnace, blast furnace inner volume can be reduced while maintaining the same production rate. Because the downsized oxygen blast furnace makes it possible to relax burden material strength requirements, energy consumption for agglomeration in the coke oven and sintering machine can also be reduced. Therefore, a DEM simulation was carried out to confirm the effect of the burden load reduction in the downsized condition. It was found that the compressive stress in the downsized oxygen blast furnace was 20–30% less than that in the conventional blast furnace. The energy flow in the ironmaking process was also investigated by using a material and energy balance model, considering the functions of an integrated steel works. It was found that the energy consumption of the ironmaking process based on the energy saving oxygen blast furnace could be reduced by 5.3% while maintaining the same energy supply to downstream processes.
In order to analyse the influence of sinter bed on the SO2, the sulphur distribution in the sintering process is studied, which indicates that the sulphur enrichment mainly at the preheating layer and drying layer during sintering process. At the wet layer, the sulphur content only is a slightly higher than the sinter mixture where the SO2 dissolve in the moisture. On the basis of thermodynamic analysis, the SO2 absorption is analysed during the sintering process, although the sintering reaction proceed at non-equilibrium condition. The major reaction that affects the residual sulphur is CaO+SO2+1/2O2=CaSO4, and the equilibrium temperature increases with the increased concentration of SO2 and O2, which could increases the sulphur residual in sinter. With the layer temperature increases, the products variation of the SO2 absorption reactions is CaSO3+CaS+CaSO4→CaS+CaSO4→CaSO4 during sintering process.
The softening and melting properties of ferrous materials are controlled by reduction degree, basicity, amount of gangue and flux, phase chemistry and their distribution in the microstructure. Design of laboratory softening-melting tests simulating blast furnace conditions greatly affects the reduction degree of the ferrous materials, since heating rate and reduction gas profile are key parameters affecting reduction degree of burden. Softening-melting behaviors of different blast furnace burden were compared by running laboratory tests under modified conditions. Comparing conventional and modified softening-melting test conditions, it can be said that modified test conditions are more practical adaptation of blast furnace conditions. SEM analysis indicated the primary mechanism for softening is due to presence of wustite as primary liquid slag former under modified test conditions, which is more close to blast furnace situation.
The wetting behavior of liquid iron and steel on Al2O3, MgO and Ti2O3 substrates was measured by using the sessile drop method. Measurements were carried out using a controlled oxygen partial pressure and using an argon protected atmosphere. For the Al2O3 and MgO substrates, reaction layers in form of FeAl2O4 and MgO–FeO (solid solution) were formed. These layers slightly decreased the contact angle and surface tension values after a full melting. For a Ti2O3 substrate in contact with pure Fe, no-reaction could be observed at the interface. Furthermore, the contact angle and surface tension values were almost stable after a full melting. For a Ti2O3 substrate in contact with steel, the contact angle and surface tension values decreased steeply after a full melting, due to the formation Al2TiO5 reaction layer formation at the interface.
For the purpose of controlling the size of non-metallic inclusions in steel, the effect of interfacial properties of alumina inclusions and molten iron on the characteristics and clustering of alumina inclusions was investigated. The interfacial properties were modified by adding different amounts of Te to molten iron at 1873 K before Al deoxidization. When the melt is not stirred after Al addition, Te was found to decrease the size of alumina inclusions and to narrow the size distribution by promoting nucleation. This effect, however, fades out with increasing holding time. When the melt is stirred for 30 s after Al addition, Te favors alumina clustering. The mean diameter of alumina inclusions, the clustering degree and the average number of particles per cluster increase with increasing Te addition from 0 to 250 ppm, thereafter decrease with further Te addition. The effect of Te addition on the clustering of alumina inclusions is discussed from the viewpoint that Te makes alumina inclusions more faceted and Te decreases the surface tension of molten iron.
TiN inclusion with large size found in ESR ingot is of great harm to steel GCr15SiMn, it is significant to elucidate the possibility of this inclusion in consumable electrode retaining in the subsequent ingot and the effect of slag composition on the content of TiN inclusion in ingot after ESR refining. Based on three remeltings and with the help of ultrahigh-temperature confocal scanning violet laser microscope, it demonstrates that TiN inclusion in electrode will completely decompose in the solid-liquid coexistence region at the electrode tip, reflecting that TiN inclusion found in ESR ingot is regenerated. For different contents of Ti in slag, there is a corresponding equilibrium value of Ti content in steel, when the content of Ti in electrode is higher than this critical equilibrium value, it will decline in steel, otherwise Ti pick-up will occur. The effect of increasing SiO2 content in slag on decreasing Ti content in steel is obvious due to the Si/Ti exchange reaction taking place, further leading to the low content of TiN in ingot.
In this work a mathematical model was developed to predict the melting kinetics of an isolated sponge iron pellet in a bath of non-reactive liquid slag. The model represents an incipient stage of modeling of a real process in an industrial scale electric arc furnace. The model was validated by computing the melting time of a material in its own metallic bath and by computing the melting time of a material of different melting point than its bath. The effect of the physical properties of the pellet and slag as well as the fluid-dynamic conditions of the metal bath were studied on both the melting kinetics of the pellets and energy consumption in the process. The study revealed that the convective heat transfer coefficient between the metal bath and the particle is an important parameter because if this value is increased, the melting time and the thickness of slag shell are drastically decreased.
Viscosity and crystallization are essential properties to characterize the lubrication and heat transfer performances of mold flux. Therefore, in this paper, the viscosity and crystallization behaviors of conventional F-containing commercial mold flux and newly designed F-free mold fluxes were investigated by using rotating cylinder method and Single/Double Hot Thermocouple Technique (SHTT/DHTT). Results shows that the viscosities of designed F-free mold fluxes are close to the F-containing mold flux; and the crystallization temperatures of F-free mold fluxes increases with the increase of basicity and Na2O/Li2O content; while it decreases with the increase of cooling rate and the addition of B2O3. The final steady state structure of F-free mold fluxes during the DHTT tests shows it is composed of a major crystalline and a thin liquid layer (5.42–7.2%) without glass phase. The results of XRD indicate that the main crystalline phases formed in designed F-free mold fluxes were calcium borosilicate (Ca11Si4B2O22) and calcium magnesium silicate (Ca14Mg2(SiO4)8. Through the comprehensive comparison, the designed F-free mold fluxes flux Sample E (Basicity 1.15, Na2O 7.92, Li2O 1.97 and B2O3 5.98) has the closest performances to the benchmark conventional mold flux Sample A.
This study addresses the effect of the cooling rate and of titanium additions on the exhibited microstructure and thermophysical parameters of thin-walled compacted graphite iron (TWCI) castings as determined by changing the moulding materials (silica sand and insulating sand LDASC), and Ferro Titanium. The research was conducted for thin-walled iron castings with a 3-mm wall thickness. The tested material represents the occurrence of graphite in the shape of nodules, flakes (C and D types, according to ISO Standard) and compacted graphite with a different shape factor and percent of nodularity. Thermal conductivity has been evaluated by the laser flash technique in a temperature range of 22–600°C. The results show that the cooling rates together with the titanium content largely influence the microstructure, graphite morphology and finally thermal conductivity of thin walled castings. Finally mechanism for thermal conductivity profile as a function of temperature for thin wall castings with different graphite morphology is provided.
This paper is concerned with the integrated optimization of finished product logistics that consists of two sub-problems: consolidation planning and transportation scheduling. In this problem, three kinds of decisions should be made simultaneously, namely the selection of candidate finished products in each warehouse (i.e., consolidation planning) and the vehicle allocation and transportation sequence of selected finished products in each warehouse (i.e., transportation scheduling). Since the loading times of products to vehicles are sequence-dependent, a tradeoff should be made between two conflicting objectives: maximization of loadage of ships and maximization of efficiency of the whole logistics. In practical industry the schedulers usually handle the two sub-problems separately, which often cause much lower efficiency of the whole logistics and higher transportation cost. Therefore, in this paper the two sub-problems are integrated and formulated as a multi-objective mixed-integer programming model, and then a two-layer multi-objective variable neighborhood search (TLMOVNS) algorithm is developed to solve it. Computational results on simulated instances show that the proposed TLMOVNS is very efficient for this problem and much superior to the current scheduling method generally used in practice.
Currently, automated inspection algorithms are widely used to ensure high-quality products and achieve high productivity in the steel making industry. In this paper, we propose a vision-based method to detect periodic defects in the surface of thick plates. To minimize the influence of a non-uniform surface property and improve the accuracy of the detection rate, a detection method based on dual-light switching lighting (DLSL) proposed. In general, single lighting (SL) methods cannot well represent the steel surface because the surface features are not uniform and strongly vary according to lighting conditions. In the DLSL method, defective regions are represented by a black and white pattern, regardless of shape, size, or orientation. Therefore, defects can be found by the black and white patterns in the corresponding images. Gabor filtering was used to find defective regions and reduce the false positive rates. To find the periodic candidates of defects, we process the period searching using manufacturing information. To identify periodic defects from among the defect candidates, we use “similarity of shapes” features with a support vector machine (SVM) classifier. The experimental results show that the proposed algorithm is effective at detecting periodic defects on the surface of thick plates.
A novel detection algorithm for strip steel defect image based on saliency map construction using Gaussian Pyramid decomposition is proposed in this paper. Firstly, the acquired gray image of strip steel is decomposed into strips steel sub-images with different resolution by Gaussian Pyramid. Secondly, the saliency map is constructed by the central-surround differences operation of strips steel sub-images and image fusion of difference sub-images. Finally, we respectively calculated mean values of maximum value in image rows and columns, in which small mean is chosen as the optimal threshold segmentation of strip image, and then to segment surface defects of steel strip. Experiment results show that the proposed method is valid for inhibition of the image background and can be realized complete segmentation and accurate detection for strip steel defect.
An invariant feature extraction method based on smoothed local binary pattern (SLBP) is proposed for strip steel surface defect images. SLBP proposed in this paper is a developed version of local binary pattern (LBP). It is determined by the sign of the difference between weighted grays in local neighborhood. SLBP has the ability of noise smoothing. In this paper, invariant features are obtained by concentric discrete square sampling template (CDSST). Firstly, defect images are resampled on CDSST by the way of coordinate mapping. Then, invariant features in scale, rotation, illumination and translation are extracted by combining two types of SLBP images and gray-level co-occurrence matrix. Experimental results show that this novel feature extraction method not only can extract features with scale, rotation, illumination and translation invariance, but also can effectively suppress noise and maintain high classification accuracy.
Prompted by the oil crisis and global warming, global steelmakers have begun to focus attention on the huge potential of hot charging and direct rolling processes for saving energy and reducing greenhouse gas emissions. New equipment and technologies have been developed in this field. In details, Japan has upgraded its reheating furnaces, applied induction heating and laser welding to endless hot rolling, developed the pair-cross mill for finish rolling and adjusted process parameters to eliminate surface defects of Nb-containing steel. Korea has also achieved endless rolling via super deformation shear joining and developed its own set of systems for the shape control of hot rolled steel products. Europe and America have designed flameless burners and reversing four-high mill stands together with their flying gage change technology and hydraulics systems for reheating and rolling. China has applied chamfered mould technology to eliminate edge and corner defects of microalloyed steel and the semi-endless rolling process to complete its intergrated production. This paper reviews development status of hot rolling and compares technologies and their effects on energy consumption, product quality, productivity and greenhouse gas emissions at global steel works.
In this paper a multi-linear regression analysis is developed to predict continuous cooling (CCT) diagrams in low carbon Nb and Nb–Mo microalloyed steels. The inputs to the analysis include the weight percentage of alloying elements, the prior austenite grain size, the retained strain and the cooling rate. To develop the model, 11 steels with different combinations of Nb and Mo were considered. In some cases, the resulting equations have been validated with external data from the literature. Additionally, the model was also employed to predict hardness and ferrite grain size with the aim of providing a tool to link microstructural features with mechanical property predictions. Both Nb and Mo additions promote a reduction of ferrite and bainite start temperatures, where the effect is more pronounced for Nb in the bainitic region. Both microalloying elements contribute to an increase in hardness and a refinement of the microstructure.
Thermomechanical treatments for manipulating grain boundary microstructure in 10 wt%Cr ferritic-martensitic steel SUH3 have been studied. Material with a high fraction of coincidence site lattice (CSL) boundaries was successfully produced and subjected to steam oxidation tests to demonstrate the utility of grain boundary engineering. Introducing a high fraction of twin boundaries in austenite resulted in a significant increase in the number of CSL boundaries along the prior austenite grain boundaries in martensite. In addition, grain boundary engineering introduced a high density of subblock structures in martensite, which resulted in a homogeneous distribution of fine precipitates in tempered martensite. Steam oxidation tests demonstrated that grain boundary engineering for SUH3 steel can achieve enhanced oxidation resistance.
A model to describe the curvature of the centerline of a strip at the delivery side during hot rolling can be used to predict the longitudinal shape of the strip after rolling, and can be useful when designing a controller to reduce camber generation. However, the existing model was obtained from studies about side-slipping, and is based on incorrect assumptions. This paper uses the definition of curvature and tangential angle to clarify why the assumptions of the former model are incorrect. Also, this paper proposes a new model for curvature of the centerline at the delivery side; this model is based on the assumption that the curved strip is generated only by the difference between the velocities of the sides of the strip. The accuracy of the proposed model is validated by FEM simulation.
Possibility of applying new roll materials (FRM (Fiber Reinforced Metal) roll materials), which are consisted of high speed tool steel reinforced with alumina fiber, to work rolls for hot rolling process is investigated by several laboratory tests. The FRM roll material expected to have superior tribological and mechanical properties, have been manufactured using sintering method (Hot Isostatic Pressing process) with which ceramic content can be increased. Wear resistance and mechanical properties and hot rolling characteristics of the FRM roll materials were investigated. As the results, it was clarified that the FRM roll material had three times or more wear resistance, a little lower rolling force and friction coefficient, and good thermal crack resistance and higher tensile strength in comparison with conventional casting roll materials such as a high speed steel roll material. Therefore, FRM roll materials can expectantly be used for hot rolling mill as high performance roll materials instead of high speed steel roll material.
This study experimentally investigated the hydrodynamics and heat transfer characteristics of a circular water jet impinging on a moving hot metal sheet as fundamental research on pipe-laminar cooling. The circular jet was issued from a 5-mm-diameter pipe nozzle. A 0.3-mm-thick sheet made of stainless steel was adopted as the test sheet. In the experiment, the liquid flow formed by the jet impingement was observed by flash photography, and the temperature profile on the underside of the moving sheet was measured by infrared thermography. The initial temperature of the moving solid was varied from 100°C to 500°C. The mean velocity at the nozzle exit ranged between 0.4 m/s and 1.2 m/s. The moving velocity of the solid was set to less than or equal to 1.5 m/s. The estimated heat flux profile on the cooled surface was found to be strongly dependent on the initial temperature of the sheet. When the initial temperature of the sheet was relatively low, a bow-shaped high heat flux region appeared in the upstream of the jet impact point. At higher temperatures, the heat flux area existed only in the jet impact regions. The heat flux increased with increasing initial sheet temperature, reached peak values, and then decreased drastically. The sharp decrease in the heat flux, which was due to the formation of a vapor layer, was influenced by the jet velocity and/or the sheet velocity.
The characteristics of dissimilar thickness dual-phase steel DP780/DP600 resistance spot welding joints have been investigated. The experimental results reveal that the weld nugget of welded joint forms as-cast structure and consists of lath martensite with little ferrite, while the heat affected zone (HAZ) near the base metal of welded joint softened. The failure location of the welded joint initiates from the DP780 in the tensile-shear test, and the proper criterion of nugget diameter to ensure the pullout failure mode should be equal to 5.5 (where t is the average thickness of both base metals in mm). The dual-phase steel spot-welding is prone to expulsion. Expulsion at the electrode/workpiece interfaces affects the surface quality; expulsion from the faying surface doesn’t decrease peak load, but increases the indentation rate of the welded joints. Compared to welding time and electrode force, expulsion is more sensitive to welding current.
The chemical and morphological features of oxides, at or near a steel surface, greatly influence the formation of the inhibition layer, critical for controlling the quality of the coating during immersion into the Zn bath. The oxides influence the wettability and the final composition of the Fe–Zn alloy coating. In this study, oxidation behaviors of Advanced High Strength Steels (AHSS) are studied during annealing in a H2-5%N2 atmosphere with controlled dew-points (0 to –40°C) and gas flow rates (5 to 100 l/min.). Compact external oxides, with a high SiO2, Al2O3 ratio, were found to form on the steel surface when the dew point was low and/or the gas-flow rates high. As dew points were increased and gas-flow rates decreased the surface oxides became less continuous and the presence of internal oxides became substantial. The influence of gas-flow rate cannot be attributed solely to the gas phase transport of oxidants to the metal surface and it is suggested that the equilibration state of the gas and consequent amount of molecular oxygen present near the surface needs to be considered.
The effect of REM (Ce,La,Nd) addition on austenite grain refinement in the heat affected zone in Fe-0.07/C-0.05/Si-1.5/Mn-0.003/S (mass%) steel was investigated by microstructural observation by SEM/EDS and thermodynamic analysis. It was found that the 30 ppm of REM addition is most effective for reducing the austenite grain size in annealing at 1450°C for 10 sec, but the grain size increases with increasing REM contents of more than 30 ppm. Since the total numbers of precipitates of oxides, sulfides and oxysulfides increase with increasing REM contents, this austenite grain growth cannot be explained by Zener’s pinning model. A thermodynamic calculation showed that REM oxides and MnS in oxysulfides are formed by the miscibility gap in 30 ppm REM steel, which results in the formation of liquid MnS-rich precipitates due to eutectic reaction. On the other hand, the melting temperature of MnS-rich precipitates increases with increasing REM in oxysulfides, and the solid precipitate particles are not effective for inhibiting grain growth. It is suggested that austenite grain growth is suppressed by the liquid phase pinning effect of MnS-rich precipitates.
The strength of pearlitic steel was clearly reduced by annealing, even though cementite stably maintained a lamellar structure. In response, lattice strain of the ferrite phase in pearlite monotonically decreased with increasing annealing time. As a result, a good linear relationship was established between the strength and ferrite lattice strain independent of the interlamellar spacing and morphology of cementite. This suggests that the ferrite/cementite elastic misfit strain contributes to the high strength of pearlitic steel.