High casting speed is the hot research topic in the field of slab continuous casting with the direct effect on equipment design, manufacturing processes, product quality, cost control and energy saving and consumption reducing. In comparison to the level of high casting speed in abroad, the technical index in domestic plants is still behind. However, some researchers succeeded in exploring high speed slab continuous casting and improved the casting speed to higher than 1.5 m/min, even over 2.0 m/min ranking as the advanced level. High speed slab continuous casting is actually a systematical engineering, with great demand on the supporting technologies to make it run at high efficiency, high safety and high stability. The tailored design and optimization should be carried out including submerged nozzle structure parameters, mold oscillation, mold flux, mold cooling, flow control by electromagnetic field and level fluctuation compensation. The results in previous studies show that the key factors influencing the product quality and safety and stability in high speed continuous casting are the steady flow and heat transfer of liquid steel in the mold, together with the even growth and good lubrication of the shell. Low carbon steel and hyper-peritectic steel are preferentially employed in the trial of high speed continuous casting, and the casting speed of hypo-peritectic steel and high carbon steel is still falling behind, although it increases obviously. The definition of high speed continuous casting is different with the variation of steel grade, strand type, equipments, operation, and technology, and the level of high casting speed is also different in different era. High speed mold metallurgy technologies are the core guarantee for high efficiency slab continuous casting. It is expected that the contents and conclusions of the study can be the theoretical and technological references for the experts and researchers regarding the issue of high speed continuous casting.
The zero reforming of coke oven gas and direct reduced process is of great significance for the green transformation of steel enterprises, with significant energy-saving and emission reduction effects. Due to the high composition of φ（H2）/φ（CO） and a certain amount of CH4 in the reduction gas of this process, the reduction atmosphere conditions of this process were simulated, and the effects of different conditions on the metallurgical properties indicators such as reducibility, compressive strength, reduction swelling index, and whole ball index during the reduction process of pellets were explored. This is of great significance for the smooth operation and energy conservation of this process. The results indicate that, as the reaction temperature increases, the reduction degree, carbon content, and swelling index increase, while the compressive strength and whole ball index decrease. After the reduction temperature increased from 850 ℃ to 950 ℃, the increase in carbon content leads to severe expansion and fragmentation of the pellets in the later stage of the reduction reaction, and the reduction degree corresponding to the maximum swelling index changes from 40%-50% to about 60%. After replacing CO with an equal amount of H2, in the early stage of the reaction, due to the increase in reduction rate, the swelling index gradually increases, while the compressive strength and whole ball index decrease. In the later stage of the reaction, due to the inhibitory effect of H2 on the growth of iron whiskers and carbon deposition, the swelling index of the pellets decreases, while the compressive strength and the whole ball index increases. By using equal amounts of N2 to replace CH4, the reduction degree, carbon content, and swelling index of the pellets decrease, while the compressive strength and whole ball index increase. After the reduction degree of the pellets exceeds 40%, the range of changes in various metallurgical properties with the change of φ（CH4） gradually increases. When the reduction time of pellets under various conditions is 20-40 min and the reduction degree is 60%-80%, the corresponding changes in metallurgical properties reach their extreme values. During the production process, attention should be paid to the indicators of pellets in this area. The carbon evolution process of CH4 has a certain deteriorating effect on the metallurgical properties of pellets, and φ（CH4） in the vertical furnace should be controlled.
In response to the issues of lower temperature at the top layer of thick layer sintering leading to raw materials and excessive heat accumulation in the lower layer causing over melting, a sintering scheme with carbon distribution at the top, upper and lower layers was proposed. Based on numerical simulation, the heat and mass transfer characteristics within the material layer under a three-layer carbon distribution scheme were investigated. The spatiotemporal evolution patterns of heat and mass transfer characteristics were explored when the coke ratio varied in each layer under the three-layer carbon distribution scheme. The numerical simulation results indicates that the temperature of the upper layer during the sintering process is determined by the heat release from coke combustion and the heat exchange between the cold air and the material layer. The amount of coke in the top and upper layer has a substantial impact on the evenness of the radial temperature distribution in the material layer. By increasing the coke ratio in the upper layer, the formation of a high-temperature zone in the upper burden layer can be enhanced. This leads to improved uniformity in the radial temperature distribution and helps alleviate the temperature drop caused by the intake of cold air during the initial sintering stage. This adjustment has minimal effect on the subsequent molten volume index. Excessive increase in the coke ratio in the upper layer can significantly elevate the maximum temperature and melt of the upper layer, thus improving its heat storing capacity. Simultaneously, this may result in an increase in the molten volume index in the lower burden layer, but there is a risk of over melting. Reducing the amount of coke at the top and raising the coke ratio in the upper layer while keeping the overall coke ratio of the material layer constant will shorten the duration of coke combustion at the top. Following the burning of top coke, the oxygen content in the lower region of the upper burden layer rises, hastening the sintering process. As a result, the total maximum temperature, pressure drop, and molten volume index decrease, reducing the sintering impact. Keeping the total coke ratio of the material layer constant, reducing the coke ratio in the upper layer and extending the length of the coke-rich zone in the upper layer has a minor impact on the sintering effect of the burden layer. However, lowering the coke ratio in the lower layer and raising the coke ratio in the higher layer efficiently promotes sintering in the upper burden layer, decreases the maximum temperature of the lower burden layer, and prevents over melting. Simultaneously, the radial temperature distribution uniformity of the load layer and the molten layer volume index improve.
The iron and steel industry is facing the problem of depletion of high-quality coal carbon resources and green carbon reduction, combined with the uneven distribution of coal resources in China, the large proportion of bituminous coal reserves and the global "carbon peak, carbon neutral" development background. The injection of high proportion bituminous coal can make full use of resources, alleviate the smelting cost of enterprises, and reduce the carbon emission of blast furnace smelting, which will become the mainstream trend in the future. In order to investigate the effect of high proportion bituminous coal injection on the process of blast furnace, a mathematical model of high proportion bituminous coal injection in blast furnace was established based on the production data of large blast furnace and the material balance and heat balance. Through theoretical calculation, the variation law of theoretical combustion temperature, gas content, direct reduction degree, carbon and heat distribution and carbon dioxide emission in blast furnace with the increase of bituminous coal injection ratio was analyzed. The results show that when the proportion of bituminous coal is increased by 10 percent point, the theoretical combustion temperature decreases by about 6-7 ℃, the direct reduction degree decreases by about 0.012, and the gas content in the furnace bosh increases by about 5 m3. Regional carbon, including the total carbon per ton of iron into the furnace, the carbon consumption of direct reduction of iron, the carbon consumption of desulfurization, and the carbon consumption of burning before the tuyere, all show a decreasing trend. The effect of heat distribution in the furnace is obvious. In terms of heat income, the heat release of carbon combustion decreases, the heat release of C and H elements increases, the physical heat brought by hot air decreases, and the overall heat income shows a downward trend. In terms of heat expenditure, desulfurization heat consumption decreases, coal decomposition heat increases, iron oxide decomposition heat consumption is basically unchanged, total heat expenditure is basically unchanged, and the whole furnace heat loss shows a downward trend with the increase of bituminous coal proportion. At the same time, the theoretical combustion temperature and heat loss decrease caused by the increase of bituminous coal ratio by 10 percent point, and the oxygen enrichment rate of 0.3 percent point and the coal ratio of 2.1 kg need to be increased at the same time to maintain the normal condition of the blast furnace. After blast furnace injection of high proportion bituminous coal, the carbon dioxide emission shows a decreasing trend, and the reduction of ton iron is about 3-5 m3.
The bosh, belly and lower part of the shaft the furnace body area is one of the limitations of efficient and longevity operation of the blast furnace, the formation of a stable slag crust is the key to ensuring the safety of the cooling stave operation in this area. In order to explore the essential characteristics of slag crust, firstly, the slag crust samples were obtained from the bosh, belly and lower part of the shaft through the anatomy and the investigation of cooling stave breakage in several blast furnaces. Secondly, the phase composition and microstructure evolution of the slag crust were studied through analysis methods such as XRF, XRD, and SEM-EDS. Based on the compositional characteristics of slag crust, a category system of slag crust was established, focusing on comparing the phase and morphological characteristics of cooling stave slag crust of blast furnace with different volumes and different cooling stave materials. Finally, the cooling stave slag crust regulation measures were formulated. The results show that the cooling stave slag crust can be mainly divided into two categories, the slag crust mainly composed of slag-iron and the harmful element binder mainly composed of ZnO and carbon. Among them, harmful element binders have poor thermal stability and are prone to detachment, which is not conducive to the safe operation of the cooling stave. The basicity of the slag in the slag crust first increases and then decreases with the decrease of height. The composition of cooling stave slag crust of blast furnace with different capacity under the same cooling stave material does not differ much, while the difference of slag crust of blast furnace body with different cooling stave materials is large. The copper stave slag crust has a clear layered structure, and its slag composition shows the characteristics of high aluminum and low magnesium slag system, the main precipitation phase is Ca2Al2SiO7. While the cast iron stave slag crust has no layered structure, the content of Al2O3 and MgO is slightly lower than that of blast furnace slag, and its main precipitation phase is Ca3MgSi2O8. Finally, it is clarified that the formation and control of slag crust should be carried out from the aspects of edge gas flow temperature and slag composition, in order to ensure the crystallization kinetics conditions of slag hanging on the surface of the cooling stave and provide a theoretical basis for the long-term operation of the cooling stave of the blast furnace.
In order to study the characteristics and formation mechanism of inclusions in the smelting process of Fe-Cr-Ni corrosion-resistant alloy, industrial experiments were conducted to sample the entire process of 8810 nickel based alloy. Scanning electron microscopy （SEM） and energy dispersive spectroscopy （EDS） were used, and thermodynamic and kinetic calculations were combined to explore the characteristics and formation mechanism of large-sized inclusions. The research results indicate that the large-sized inclusions appearing in steel can be mainly divided into two types. One type is the micro inclusions of low melting point SiO2-CaO-Al2O3-MgO large particles containing mass percent of SiO2 bout 37%-45%, another type is low melting point CaO-Al2O3-MgO large particle micro inclusions without SiO2. By comparing the composition of inclusions and slag, as well as calculating the size of slag inclusions using a model, it was verified that both types of inclusions were formed due to the inclusion of slag during the AOD stage steel stirring. However, during the AOD reduction period, the former was either removed or modified as CaO-Al2O3-MgO inclusions due to the recreation of slag and the increase of Al content. In the statistical analysis of inclusions, it is indicated that, the CaO-Al2O3-MgO inclusion is more severe than the SiO2-CaO-Al2O3-MgO inclusion in the subsequent process, and it still exists until the casting period. Therefore, the behavior of low melting point CaO-Al2O3-MgO inclusions in subsequent smelting was analyzed through dynamic calculations and Thermo Calc software. The results show that the inclusions are not easy to float and remove from the appearance of AOD tapping until the end of LF hanging ladle. At the same time, it leads to the precipitation of TiN with CaO-Al2O3-MgO inclusions as the core during the solidification stage of mold casting, forming composite inclusions, as well as single TiN inclusions appearing during the solidification stage, which ultimately become the main types of inclusions in the ingot.
Due to its good mechanical properties and cutting performance, sulfur-containing free-cutting steel is widely used in industries such as automotive manufacturing, machinery manufacturing, and shipbuilding industry. With the development in the fields of infrastructure, passenger vehicles, and maritime transportation in recent years , the production volume and quality requirements of sulfur-containing free-cutting steel have also increased. The method of calcium treatment is usually used to regulate the inclusions in the steel. However, the sulfur-containing characteristics of sulfur-containing steel have a significant negative effect on the cleanliness and castability of the steel during the smelting and casting processes. Therefore, establishing a reasonable calcium treatment process is of great importance for improving the castability of sulfur-containing free-cutting steel. The effects of factors such as the timing of sulfur wire addition, the time interval between feeding calcium wire and sulfur wire, and the amount of calcium wire added on the cleanliness of the steel and the clogging of nozzle were systematically investigated, analyzing the main reasons for the nozzle clogging in sulfur-containing free-cutting steel. The research shows that inclusions with CaS on the surface and Al2O3 in the core are generated in the steel after feeding sulfur wire and calcium treatment. The continuous deposition and adhesion of these inclusions on the inner wall of the nozzle are the main cause of nozzle clogging. In steel with w （[Al]）=0.03%, when w （[S]） exceeds 0.015%, it is prone to the formation of high melting point inclusions, deteriorating the castability of the steel. For 140 t of 45S sulfur-containing free-cutting steel, feeding the sulfur wire separately after LF and RH, extending the feeding interval between calcium wire and sulfur wire to over 10 minutes, and reducing the total feeding amount of calcium wire to below 100 m can effectively reduce the quantity of CaS·Al2O3 inclusions in the steel. This can increase the number of continuous casting heats for 45S steel to over 15 heats. It provides a theoretical basis for optimizing the calcium treatment process of sulfur-containing free-cuttingsteel, which helps improve production efficiency and product quality. It is of great significance for promoting the application and development of sulfur-containing free-cutting steel in the fields of machinery, transportation, and other industries. Future research will focus on how to control the quantity and morphology of inclusions in sulfur-containing steel, the impact of inclusions on the quality of steel plates after rolling, and how to reduce the amount of calcium added.
The medium carbon micro-alloyed steel containing niobium is crack-sensitive steel, which is easy to crack in the continuous casting process and seriously deteriorates the surface quality of the bloom. The causes of corner cracks in 46MnVNbS5 steel were analyzed using an optical microscope, scanning electron microscope with energy dispersive spectrometer, three-dimensional etching technology, high temperature confocal laser scanning microscope and Thermo-Calc software. The surface crack distribution of 320 mm×425 mm casting bloom was observed, and no obvious cracks were found in the range of 0-20 mm of the inner arc, and only pitted defects were observed on the samples 20 mm away from the inner arc. There are obvious cracks in the range of 20-40 mm, and the crack size is generally greater than 10 mm. At the distance of 25 mm from the inner arc surface, cracks were observed only at the low multiplier surface. At 30 mm, 35 mm, and 40 mm from the inner arc surface, there were cracks on multiple observation surfaces of the sample. The crack grows along the dendrite growth direction. A large number of coupling precipitates of manganese sulfide and carbonitride in and near cracks were determined by SEM-EDS. The surface of the crack specimen was corroded and it was found that the crack spread along the ferrite film. The thermodynamic results show that manganese sulfide is the main inclusion in the solid-liquid zone of steel. The niobium-rich carbonitride and vanadium-rich carbonitride will be precipitated successively, which is consistent with the types of inclusions obtained by experiments. The cracking zone （620.1-794.3 ℃） of steel containing niobium was determined by high-temperature confocal laser microscopy, and numerical simulation was applied to simulate the corner temperature field of continuous casting bloom, and the cooling system was optimized to avoid the cracking zone of medium carbon micro-alloyed steel containing niobium. This study provides the methods to solve the crack of casting bloom and has important practical significance.
The evolution of microstructure and cementite in the process of cold-drawing pearlitic steel wire has an important effect on the properties of the completed steel wire. The SEM, TEM, physicochemical phase analysis and APT were used to study the organization evolution and carbide evolution behavior of 82B-V pearlitic wire rods during the drawing to produce steel wires and strands. The microstructure results show that after the cold drawing process, the pearlitic lamellae gradually turns to drawing direction, and the lamellae spacing decreases from 140 nm to 80 nm, showing good deformation ability. During drawing process, a large number of dislocated cellular structures are generated within the ferrite in the steel wire. As a result, the ferrite lamellae becomes partitioned by dislocation walls, exhibiting a distinctive "bamboo" morphology. The cementite rotates and part of cementite phases between the regions of neighboring ferrite lamellae dissolve and disappear. The results of physicochemical phase analysis show that after drawing of wire rod into steel wire, element C in the alloy cementite diffused into the ferrite and a part of cementite dissolves, resulting in a reduction of the mass fraction from 7.99% to 6.67%. The content of alloy cementite in the stabilized strand is slightly recovered and increased to 7.00%; and the results of APT show that, compared with the wire rod, the average concentration of C atoms in the cementite of steel strand is only 13.5%, reduced by 7.5 percent point. The average concentration of Cr, Mn and V atoms in the strand cementite is 0.51%, 1.59% and 0.23%, which decreases by 0.246, 0.785 and 0.170 percent point, respectively, which further confirms that the dissolution of alloy cementite occurred in the process of large deformation and drawing and the diffusion of C, Cr, Mn and V into the ferrite. Through the tensile experiments and fracture morphology analysis, steel wire in the tensile process without necking produced, the fracture shows shot and crystalline, there is a tear prism through the entire cross-section. While the steel strand produced necking, the fracture is a gray lusterless fibrous, the heart of the tear prism is shallower, and the crystalline morphology is significantly reduced the strand showed better overall mechanical properties, showing a better overall mechanical properties, its tensile strength and section shrinkage reaches 2 045 MPa and 35.2%, respectively.
The nickel in 40CrNiMo steel is mainly used to improve low-temperature toughness, but the price of nickel is expensive and does not meet the requirement of low cost. A low cost CrNiMo steel with a mass fraction of 0.08% vanadium microalloyed and nickel content of only 0.3% was developed to replace 40CrNiMo steel （the mass fraction of nickel was about 1.5%）. The differences of matrix and carbide between test steels were studied by means of SEM, EBSD, TEM and XRD. The tensile test and -40 ℃ impact test were taken to compare the toughness of test steels under the same strength（10.9 grade）. The results show that the prior austenite grain size of 40CrNiMo steel is smaller than that of developed steel, which is （14.5±5.3） μm and （20.6±7.1） μm respectively. The large angle grain boundary density of 40CrNiMo is 0.7/μm high than that of the developed steel due to the fine grain size. Due to the tempering resistance improving effect of vanadium, the tempering temperature of the developed steel is increased by 60 ℃ compared with 40CrNiMo steel at the same tensile strength, and the recovery degree of the matrix is improved, thus obtaining a higher proportion of large angle grain boundary and a lower dislocation density. The dislocation density of the two test steels is 9.3×1014/m2 and 2.3×1015/m2, respectively. The MC, M2C and M3C carbides are precipitated in the developed steel microstructure after tempering. The MC carbides are spherical with diameter less than 20 nm, while the M2C and M3C carbides are ellipsoidal with maximum length less than 150 nm. The M2C and M3C carbides are precipitated in 40CrNiMo steel. The M2C carbides are ellipsoidal with maximum length of less than 100 nm, and the M3C carbides are strips with length of more than 500 nm. Through the statistics of maximum size of all carbide in observation fields, it is found that the proportion of M3C cementite in developed steel is lower, and the overall size of carbides is smaller. Although the prior austenite grain size of the developed steel is large and its large angle grain boundary density is low, but its sufficient recovery and fine spherical carbide particles got after tempering improve its low-temperature toughness. When the tensile strength is about 10.9 grade, the impact absorb energy of the developed steel reaches 86 J at -40 ℃, which is higher than 74 J of the 40CrNiMo steel.
In order to meet the diversified needs of high-strength dual-phase steel users in automobile manufacturers, two kinds of high-strength dual-phase steels with different microstructure characteristics of F/M and B/Ar were obtained, adopting reasonable chemical composition design and process control. The process design principles and microstructure characteristics of F/M and B/Ar high-strength dual-phase steels were studied by means of SEM, TEM, tensile and hole expansion tests, and the influencing factors of mechanical properties were analyzed. The results show that the annealing temperature of F/M dual-phase steel is 810 ℃ in the range of （AC1+AC3）/2±8 ℃（AC1 and AC3 are the start and end temperatures of austenite transformation during heating, respectively）, which is mainly composed of 47% ferrite, 46% martensite and 7% block retained austenite. The ferrite has two forms of recrystallized ferrite and proeutectoid ferrite, and the grain size is 2.5-4.0 μm and 1.0-2.5 μm, respectively. The annealing temperature of B/Ar dual-phase steel is set to 900 ℃ in the single-phase austenite region. The microstructure is mainly composed of 84% bainite and 16% second-phase retained austenite. The bainite is based on the original γ grain. The phase transformation is formed, and the retained austenite is characterized by lamellar or discontinuous block. There are obvious differences in microstructure morphology between F/M and B/Ar dual-phase steels. The coordinated deformation effects of each phase structure are different during the deformation process, which affects its mechanical properties. The tensile strength can be controlled at the level of 1 000 MPa by both processes. The nano-sized VC precipitated phase particles with a diameter of 4-13 nm are dispersed in the matrix, and the precipitation strengthening amount exceeds 220 MPa, which interacts with high-density dislocations, and finally improves the strength and plasticity of the material. The tensile strength of F/M dual-phase steel is 1 035 MPa, and the elongation after fracture is 18.7%. Compared with F/M dual-phase steel, the tensile strength of B/Ar dual-phase steel increases by 111 MPa to 1 146 MPa, and the product of strength and plasticity reaches 19.83 GPa·%. It has the characteristics of high yield ratio and hole expansion rate, which are 0.709 and 38%, respectively.
Ultra-high strength stainless steel is widely used in aviation, aerospace and other fields because of its good comprehensive performance. A new type of 2.1 GPa grade ultra-high strength stainless steel was taken as the research object, and the influence law of different solid solution temperature and aging treatment on the mechanical properties of steel was investigated. Scanning electron microscope （SEM）, X-ray diffractometer （XRD）, transmission electron microscope （TEM） and other means were used to characterize the microstructures of the steel in solid solution and aging state, and an intrinsic correlation was established between the heat treatment-mechanical properties-microstructure. The results show that the solid solution organization of the steel is slat martensite + residual austenite, the size of the original austenite grain increases with the increase of solid solution temperature, the strength shows a decreasing trend with increasing of solid solution temperature, while the impact absorption work increases firstly and then decreases. The strength of the solid solution by 1 050 ℃ is the highest, but due to the un-dissolved M6C phase disrupts the continuity of matrix organization resulting in low impact absorption work, the strength and the impact absorption work of solid solution by 1 100 ℃ are lower than the 1 080 ℃ solid solution. Although the former high austenite content is conducive to improving toughness, coarse grains lead to a reduction in the impact absorption work. After aging, the strength of the steel significantly improved, 1 050 ℃ solid solution + aging has the lowest strength, while 1 080 ℃ solid solution+aging has the highest strength, the tensile strength is 2 161 MPa and yield strength is 1 784 MPa. Plastic toughness also better than 1 050 ℃ and 1 100 ℃, 1 080 ℃ impact absorption work is 37.5 J, which has the best toughness match, and there are a large number of small, diffuse Laves phase and M2C phase precipitated in the slat martensite, which is the main reason for obtaining ultra-high strength. In the martensite slat boundary, the thin film austenite is the key to maintain good toughness. The research steel grade is the highest strength level of stainless steel in the international arena currently, and the research results can provide data support to enhance the maturity of its engineering technology.
30Cr15Mo1N is a kind of strength martensitic stainless steel with nitrogen mass fraction of 0.4%, the important problem in its application is the matching of high hardness and high corrosion resistance. The microstructure and corrosion resistance of 30Cr15Mo1N steel after tempering with different temperatures were studied by SEM, TEM, chemical precipitates analysis and salt spray test. The results show that the microstructure after quenching, cryogenic treatment and tempering with temperature below 500 ℃ is distributed of micrometer undissolved precipitates and nanometer newly precipitated precipitates on the lath martensite matrix. With the increase of tempering temperature, the types of precipitates remain unchanged, which are M23C6 and Cr2（C,N） types, and the number and size of precipitates increase gradually. In particular, when tempering at 475 ℃, the precipitated phase mass fraction suddenly increases, the martensite gradually dissolves and the tempering sorbite occurs when tempering temperature is higher than 500 ℃. At 500 ℃, the peak hardness of secondary hardening is 60.7HRC. With the increase of tempering temperature, the strengthening effects of martensite and solid solution gradually decrease, and the secondary hardening effect mainly comes from the strengthening effect of a large amount of precipitated phases. The salt spray test results show that small pitting corrosion is the main feature when the tempering temperature below 400 ℃, and visible corrosion occurs after tempering at 450 ℃. When the tempering temperature is no less than 450 ℃, granular and lamellar corrosion products are deposited on the specimen surface. The samples tempered at 475-500 ℃ have the most serious corrosion, and the corrosion resistance deteriorates as the tempering temperature increases. During the tempering process, the precipitation of Cr containing precipitates leads to Cr-depletion zone in the surrounding matrix, which leads to the decline of corrosion resistance. When the tempering temperature reaches 600 ℃, the distance between the Cr depletion on the edge of the precipitates increases, and the corrosion resistance is slightly improved. Therefore, if high hardness and high corrosion resistance coexist, meet the hardness requirements of bearings no less than 58HRC, tempering temperature should not be higher than 400 ℃.
Steel bridges in coastal environments may experience pitting corrosion in the early stages of corrosion, and the appearance of corrosion pits can easily exacerbate the local stress concentration and the deterioration of structural bearing capacity, which affecting the safety and reliability of steel bridge service. Q345qD bridge structural steel was taken as the research object, the samples were designed and a 64 h full immersion corrosion test was conducted, the surface morphology point cloud data of the corrosion samples was collected through the KathMatic laser microscopy, and a high-precision method was proposed for extracting corrosion pits and their geometric parameters based on point cloud data. The statistical filtering algorithms was used to smooth the point cloud data, the Random Sample Consensus （RANSAC） algorithm was used for planar segmentation of corrosion pit point clouds, the Density-Based Spatial Clustering of Applications with Noise （DBSCAN） algorithm was used to cluster the corrosion pits and obtain the key data of corrosion pits. The Alpha Shape algorithm and Graham algorithm were used for corrosion pit extraction and labeling. The extraction results show that the Alpha Shape algorithm can accurately obtain the depth and the surface area of the corrosion pits. Compared with Graham algorithm, the accuracy of extracted pit surface area using Alpha Shape algorithm is overall improved by 22.39%. The statistical analysis was conducted on the depth and surface area of corrosion pits, the distribution empirical functions were fitted, and the distribution hypothesis tests were done. The results show that at a significant level of α=0.05, the corrosion pit depth of Q345qD steel samples at the pitting stage conforms to the Gumbel, Logistic and Weibull distribution, and the correlations is 99.10%, 96.23%, and 99.02%, respectively, while the corrosion pit surface area follows the Logistic distribution, with a correlation of 97.02%. This method can provide reference for the research of pitting corrosion distribution of Q345qD steel in the same environment and the decay of mechanical properties of Q345qD steel under random pitting corrosion.
The development model of "solid waste does not leave the factory", implemented by the iron and steel industry, requires that all solid waste generated during the production process be consumed and utilized within the factory. This model presents a challenge due to the complex composition of various types of sludge produced in iron and steel plants, including iron oxide mud, OG coarse grain, calcium carbonate sludge, acid-base neutralization sludge, continuous casting sludge, and biological sludge. These sludges contain valuable elements such as iron and carbon, and the sintering process offers an effective method for recycling this solid waste. The recycling of various sludges from a domestic iron and steel enterprise using sludge heating combustion and ore blending tests was investigated. The feasibility and optimal sludge addition ratio in the sintering process were assessed by analyzing the composition of flue gases and the quality indices of sintered minerals. The results indicate that sludge heating combustion leads to the formation of pollutants such as SO2, CO, HCl, NO2, NO, CH4, and VOCs, with particularly high emissions of CO and SO2, which reached 16.94 and 12.10 mg/g（sludge）, respectively. VOCs emissions are measured at 2.58 mg/g（sludge）. Furthermore, when sludge is blended with ore for sintering, there is a significant increase in the emission concentrations of SO2, NO, CH4, and VOCs. With a 5% sludge addition ratio, the emissions per ton of sinter of these compounds increase by 143%, 44%, 219%, and 1 577%, respectively, compared to baseline tests. The yield and quality index of the sinter initially improve but begin to decline as the sludge addition ratio increases from 0% to 7%. Notably, when the sludge addition ratio increases from 0 to 3%, both the sintering utilization coefficient and the vertical sintering speed improve. However, further increases in sludge addition leads to a decrease in the quality index of sintering production. Therefore, to avoid an increase in the emission concentration of flue gas pollutants and a decline in production quality, it is advisable to control the sludge addition to within 3% of the mass fraction of the sintering raw materials, especially when the capacity of the sintering flue gas purification system is not fully utilized.
The size and morphology of sulfides are important for both machinability and mechanical properties of free-cutting steels. In order to improve the level of sulfide control, characteristics （length, aspect ratio and area） of sulfide in Ca treated free-cutting steel before and after hot rolling were analyzed systematically by scanning electron microscope equipped with an automatic inclusion analyzing system. There were two kinds of sulfides in Ca treated free-cutting steel, pure MnS and CaS-MnS. The proportion of CaS-MnS was 13%-21%, and the core was an oxide. The energy spectrum distribution indicated that CaS-MnS was a solid solution phase with uniform composition. For pure MnS, the average and maximum aspect ratios in continuous casting billets were 2.191 and 26.87, respectively. After hot rolling, the average and maximum aspect ratios of pure MnS were 4.583 and 60.83, respectively. The average aspect ratio increased by 107.12%. For CaS-MnS, the length, aspect ratio and area in continuous casting billets were significantly smaller than that of pure MnS. After hot rolling, the deformation of CaS-MnS was slight, with an average aspect ratio of 1.598 and a maximum aspect ratio of 14.42, respectively. The average aspect ratio increased by 8.48%. The area had a significant impact on the deformation of pure MnS. For MnS with area no more than 30 μm2, the aspect ratios in both continuous casting billet and hot rolled rod were relatively small （<4）. For MnS with area larger than 30 μm2, the aspect ratio significantly increased after hot rolling. The formation mechanism of sulfides was explained. The inclusions after Ca treatment were CaO-Al2O3. After sulfur addition, CaO was transformed into CaS because the S content in the steel was much higher than that of O content. CaS and MnS had mutual solubility, which can promote the heterogeneous nucleation of MnS and form fine CaS-MnS. At the end of solidification, the supersaturation of Mn and S increased sharply, which makes a large amount of MnS to precipitate in a short period of time. It helps to better understand the effect of Ca treatment on sulfur-containing free cutting steels and provides guidance for engineering applications.
Electromagnetic processing of materials is developing toward high-frequency technology, with high-frequency magnetic fields playing a crucial role in the material processing process.Traditional high-frequency magnetic field technology, compared to industrial frequency electromagnetic processing, can produce more significant thermal effects and electromagnetic forces on conductive materials. It is widely used in the manufacturing and processing of metal materials such as induction quenching, induction welding, induction thermal bending, induction overlay, and electromagnetic suspension melting. This technology offers advantages in efficiency, controllability, and low energy consumption. In recent years, new applications of high-frequency magnetic fields have been developed to act on low electrical conductivity liquids, breaking through the limitations of traditional electromagnetic technology in processing low-conductivity materials. High-frequency traveling wave magnetic fields generated by uniquely arranged dual-phase induction coils can produce driving effects on low-conductivity liquids with electrical conductivity between 1-100 S/m at a velocity on the order of centimeters per second. This enhancement can intensify the three-transfer processes in industrial applications such as liquid steel slag, with potential applications in hot state steel slag iron extraction technology to improve the recovery rate of iron resources from steel slag. By combining the characteristics and applications of traditional high-frequency technology and high-frequency traveling wave magnetic field technology, it points out that the future development of high-frequency magnetic field technology in the field of material processing should achieve complex processing procedures through coil structure design and frequency control, and expand applicable materials to low-conductivity media.
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