Journal of Iron and Steel Research(International)最新文献

The influence of explosive charge thickness on the quality of explosive welding of dissimilar metals was investigated. The lower limit law should be followed in the course of explosive welding. Three welding experiments of stainless steel (410S) and steel (Q345R) were carried out in three different kinds of explosive charge thicknesses, namely 15, 25 and 35 mm. Interfaces of morphology and mechanical properties of three samples were observed and tested. It was found that micro and small wavy bonding is mainly formed for charge thickness of 15 mm whose strength is the highest with minor deformation and few defects in the interface; small and middle wavy bonding are mainly formed for charge thickness of 25 mm whose strength is comparatively mediocre; big wavy bonding is mainly formed for charge thickness of 35 mm whose strength is the lowest. The cause of high bonding strength of the micro and small wavy interface was analyzed and verified on the basis of the results of Electron Probe Micro-Analyzer (EPMA) tests of three selected samples.

The thermal stability of retained austenite (RA) and the mechanical properties of the quenched and intercritical annealed 0.1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained austenite during tempering within the range from 200 to 500 °C were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and tensile testing. The results showed that there was a slight decrease in the RA volume fraction with increasing tempering temperature up to 400 °C. This caused a slight increase in the ultimate tensile strength (UTS) and a slight decrease in the total elongation (TE); thus, the product of UTS to TE (UTS×TE) as high as 31 GPa • % was obtained and remained nearly unchanged. However, a portion of the RA began to decompose when tempered at 500 °C and thus caused a ∼35% decrease of the RA fraction and a ∼16% decrease of the value of UTS×TE. It is concluded that the ultrafine lamellar duplex structure is rather stable and the excellent combination of strength and ductility could be retained with tempering temperature up to 400 °C. Thus, thermal processes such as galvanization are feasible for the tested steel provided that their temperatures are not higher than 400 °C.

Lots of work has been done to investigate slab surface microstructure evolution during continuous casting in order to improve hot ductility and avoid transverse cracks. The slab surface microstructure after continuous casting was characterized by optical microscopy, and the precipitation behavior was investigated by transmission electron microscopy. At the same time, the mechanical properties of the slabs were measured using a Gleeble 1500D thermal simulator and the transformation temperatures were examined by means of a thermal dilatometer. The experimental results show that homogeneous microstructure without film-like ferrites and chain-like precipitates at grain boundary can be obtained through surface intensive cooling and transverse cracks do not occur on the slab surface. For the experimental steel, fine ferrite can form at slab surface when the water flow rate is larger than 1560 L/min at vertical section. As the distance to surface increases, microstructure turned to ferrite and pearlite. Moreover, nano-size carbonitrides precipitated in the ferrite grain and the size was larger at the junction of the dislocations. The mechanical experiment results show that the hot ductility of the sample deformed at 650 °C was better than that of the sample deformed at 750 °C. The reason is that film-like ferrite formed at the grain boundary in the sample deformed at 750 °C. Thus, the slab must be cooled quickly below A_r3 to prevent the occurrence of film-like ferrite and transverse cracks on the slab surface during casting.

According to the actual requirements, profile and rolling energy consumption are selected as objective functions of rolling schedule optimization for tandem cold rolling. Because of mechanical wear, roll diameter has some uncertainty during the rolling process, ignoring which will cause poor robustness of rolling schedule. In order to solve this problem, a robust multi-objective optimization model of rolling schedule for tandem cold rolling was established. A differential evolution algorithm based on the evolutionary direction was proposed. The algorithm calculated the horizontal angle of the vector, which was used to choose mutation vector. The chosen vector contained converging direction and it changed the random mutation operation in differential evolution algorithm. Efficiency of the proposed algorithm was verified by two benchmarks. Meanwhile, in order to ensure that delivery thicknesses have descending order like actual rolling schedule during evolution, a modified Latin Hypercube Sampling process was proposed. Finally, the proposed algorithm was applied to the model above. Results showed that profile was improved and rolling energy consumption was reduced compared with the actual rolling schedule. Meanwhile, robustness of solutions was ensured.

Cementites decomposition of a pearlitic ductile cast iron during graphitization annealing heat treatment was investigated. Fractographies and microstructures of heat treated samples were observed using a scanning electron microscope and mechanical properties were measured by a universal tensile test machine. The results indicated that during isothermal annealing at 750 °C, the tensile strength of pearlitic ductile cast iron was increased to a peak value at 0.5 h, and decreased gradually thereafter but the elongation was enhanced with the increase of annealing time. Moreover, the diffusion coefficient of carbon atoms could be approximately calculated as 0.56 µm²/s that could be regarded as the shortrange diffusion. As the holding time was short (0. 5 h), diffusion of carbon atoms was incomplete and mainly occurred around the graphites where the morphology of cementites changed from fragmentized shape to granular shape. In addition, the ductile cast iron with tensile strength of 740 MPa and elongation of 7% could be achieved after graphitization annealing heat treatment for 0. 5 h. Two principal factors should be taken into account. First, the decomposition of a small amount of cementites was beneficial for increasing the ductility up to elongation of 7%. Second, the diffusion of carbon atoms from cementites to graphites could improve the binding force between graphites and matrix, enhancing the tensile strength to 740 MPa.

The volume hypothesis, a theory about the energy scale of comminution, was adopted to analyze the degradation behavior of lump coal under different heating time. The breakage energy of chars was determined by a compression test, and the results show that the ultimate strength of chars decreased at the early stage during the heating process, resulting in a decrease of the char breakage energy. At the late stage during the heating process, the char breakage energy increased with the increase of heating time. The power consumption coefficients C_K of different chars were determined by a drum experiment, and then the char degradation behavior under different power consumptions was predicted. In addition, a gasification experiment was conducted to determine the gasification activation energy (with CO₂) of lump coal heated for different time. The results show that the gasification activation energy increased greatly at the early stage during the heating process, which showed opposite change to the breakage energy of chars. Furthermore, the internal temperature and heat changes of the bonded coal briquette were calculated by using an unsteady heat conduction equation. The large difference between the surface and the center temperatures of coal and the large amount of heat absorption at the early stage during the heating process may have a negative effect on the breakage energy of chars.

The springback of tailor rolled blanks with quenching and partitioning steels was investigated. In order to find out the springback behavior and related influence factors for the novel sheets, both experimental and simulation methods have been used to compare and analyze the springback characteristics of equal thickness blanks and tailor rolled blanks in U-channel forming. From the results, the overall springback angles of tailor rolled blanks at thin and thick sides are respectively 106. 79° and 99. 705°, which are both lower than those of the corresponding equal thickness blanks. Due to the existence of the thickness transition zone, the stress distribution in thin and thick sides of blanks is changed. The location of dangerous region in thin side of tailor rolled blanks is closer to the end of side, and the thick side moved to the middle of straight wall, which are different with the equal thickness blanks. Afterwards, the released quantitles of tangential stress and strain per unit section of blank are adopted to calculate relative springback angles and give novel evaluation criteria for qualitatively analyzing the amount of springback angles. By comparing the results, it shows that the tangential strain method is more suitable for the actual situation.

Blast furnace (BF) ironmaking is dominant for reducing pollution emission and energy consumption in iron and steel industry. Under the increasingly strict environmental pressure, some innovative technologies of BF ironmaking for environmental protection have been developed and applied in actual operating facilities. The current state of BF ironmaking in Europe, America, Japan, and China were briefly overviewed. Moreover, some innovative BF ironmaking technologies aiming at environmental harmony and operation intellectualization in the world, such as waste gas recycling sintering, BF operation with coke oven gas injection, ferro-coke, lime coating coke, BF visualization and intellectualization, were roundly summarized. Finally, some discussion on the technologies was carried out and the development trends of BF ironmaking were pointed out. The review could provide references and supports for the progress of environment-friendly technologies of BF ironmaking, thereby promoting their practical applications and achieving sustainable development of BF ironmaking, especially for Chinese ironmaking industry.

Hard coatings are used to improve the wear resistance of metals which largely depends on adhesion between substrate and coating. The wear and friction behavior of uncoated and TiCN-coated D2, M2 and M4 steels were evaluated by a pin-on-disk test under lubricated conditions. In order to evaluate the influence of lubricant on wear performance, dry friction tests were also performed. The results showed that friction coefficients were very similar for both uncoated and TiCN-coated steels. Under lubricated conditions, the uncoated D2 tool steel exhibited the lowest friction coefficient, but the TiCN-coated D2 steel presented the smallest wear rate. Abrasion was the main wear mechanism in all the tribocouples. Additionally, microhardness measurements were carried out, finding an influence of the steel substrate on the hardness of the coatings. Besides, adhesion test was conducted, suggesting a good adhesion of class 1 between substrates and TiCN coatings.

The rapid coarsening of the M₂₃C₆ carbides has been held responsible for the creep fracture in 9–12Cr martensitic heat resistant steels. A commercial P92 steel was subjected to thermal aging at a high temperature of 800 °C to investigate the ripening behavior of the M₂₃C₆ carbides. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the microstructure evolution, especially the ripening process of the M₂₃C₆ carbides. The new concept of the effective mean size, dependent on the critical radius, was introduced to correct the measured mean size and then the Ostwald theory was applied to describe the ripening behavior of the M₂₃C₆ carbides. The ripening of the M₂₃C₆ carbides was revealed to be grain boundary diffusion controlled.