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

The cavitation erosion of weld joint and base metal of China low activation martensitic (CLAM) steel in liquid lead-bismuth eutectic alloy (LBE) at 550 °C was investigated to simulate the cavitation erosion of the first wall and the nuclear main pump impeller in the accelerator driven sub-critical system (ADS). A suit of ultrasonic cavitation facility was self-designed to study the cavitation erosion. By studying the surface micro topography, roughness and mean pit depth of the tested specimens, it was found that some crater clusters and large scale cracks appeared on the tested specimen surface after the formation of numerous single craters, and the base metal exhibited much better cavitation erosion resistance than the weld bead due to the difference in their mechanical properties and microstructures. In addition, by comparing the results of static corrosion and cavitation erosion, it could be concluded that the cavitation erosion and the dissolution and oxidation corrosion in liquid LBE would accelerate mutually.

The effects of squeeze casting process on microstructure and flow stress behavior of Al-17.5Si-4Cu-0.5 Mg alloy were investigated and the hot-compression tests of gravity casting and squeeze casting alloy were carried out at 350–500 °C and 0.001–5 s^–1. The results show that microstructures of Al-17.5Si-4Cu-0.5 Mg alloys were obviously improved by squeeze casting. Due to the decrease of coarse primary Si particles, soft α-Al dendrite as well as the fine microstructures appeared, and the mechanical properties of squeeze casting alloys were improved. However, when the strain rate rises or the deformation temperature decreases, the flow stress increases and it was proved that the alloy is a positive strain rate sensitive material. It was deduced that compared with the gravity casting alloy, squeeze casting alloy (solidified at 632 MPa) is more difficult to deform since the flow stress of squeeze casting alloy is higher than that of gravity casting alloy when the deformation temperature exceeds 400 °C. Flow stress behavior of Al-17.5Si-4Cu-0.5Mg alloy can be described by a hyperbolic sine form with Zener-Hollomon parameter, and the average hot deformation activation energy Q of gravity casting alloy and squeeze casting alloy is 278. 97 and 308.77 kJ/mol, respectively.

The quantitative phase-field simulations were reviewed on the processes of solidification of pure metals and alloys. The quantitative phase-field equations were treated in a diffuse thin-interface limit, which enabled the quantitative links between interface dynamics and model parameters in the quasi-equilibrium simulations. As a result, the quantitative modeling is more effective in dealing with microstructural pattern formation in the large scale simulations without any spurious kinetic effects. The development of the quantitative phase-field models in modeling the formation of microstructures such as dendritic structures, eutectic lamellas, seaweed morphologies, and grain boundaries in different solidified conditions was also reviewed with the purpose of guiding to find the new prospect of applications in the quantitative phase-field simulations.

A numerical simulation was performed to study the flow pattern, mixing time and open-eye slag produced by argon gas injection in an industrial scale steel ladle under non-isothermal conditions. The liquid steel remains 5 min before the injection, and thermal stratification and convective flows were analyzed. Three different sequences in stages employing various argon-gas flow rates were simulated. In the first case, a sequence with the highest flow rates of argon was applied, while in the second and the third sequences, the intermediate and the lowest flow rates of argon gas were used, respectively. For determining the chemistry homogenization, the mixing time was computed and analyzed in all three cases. It was found that the cold steel is located near the walls while the steel with a high temperature is accumulated in the center of the ladle above the argon-gas tuyere. The higher and lower flows promote a faster chemistry homogenization owing to the secondary recirculations that are developed closer to the walls. The results from steel temperature drop show a good concordance with plant trial measurements.

The influence of cerium (Ce) treatment on the morphologies, size and distributions of Al₂O₃ inclusions in low carbon high manganese steel was investigated by OM, SEM-EDS and theoretical calculation. The results showed that Ce can modify the morphologies and types of Al₂O₃ inclusions. After Ce treatment, the irregular Al₂O₃ inclusions were replaced by smaller and dispersive spherical cerium oxysulfides. The effects of treatment time and Ce content on the evolution of Al₂O₃ inclusions were examined. It indicated that Al₂O₃ inclusions were wrapped by rare earth inclusions to form a ring like shape Ce-enriched band around the inclusions. Model was established to elucidate the evolution mechanism of Al₂O₃ inclusions. Evolution kinetics of inclusions was discussed qualitatively to analyze the velocity controlled step. It was found that diffusion of Ce³⁺ and Al³⁺ in solid inclusion core and the formed intermediate layer would be the limited step during the evolution process.

The microstructure evolution and impact-toughness variation of heat-affected zone (HAZ) in X80 high-strain pipeline steel were investigated via a welding thermal-simulation technique, Charpy impact tests, and scanning electron microscopy observations under different welding heat inputs and peak temperatures. The results indicate that when heat input was between 17 and 25 kJ · cm^–1, the coarse-grained heat-affected zone showed improved impact toughness. When the heat input was increased further, the martensite-austenite (M-A) islands transformed from fine lath into a massive block. Therefore, impact toughness was substantially reduced. When the heat input was 20 kJ · cm^–1 and the peak temperature of the first thermal cycle was between 900 and 1300 °C, a higher impact toughness was obtained. When heat input was 20 kJ · cm^–1 and the peak temperature of the first thermal cycle was 1300 °C, the impact toughness value at the second peak temperature of 900 °C was higher than that at the second peak temperature of 800 °C because of grain refining and uniformly dispersed M-A constituents in the matrix of bainite.

An artificial tribological layer was formed on the worn surface during sliding, through supplying MoS₂, Fe₂O₃ or their equiponderant mixtures onto the sliding interface of H13/GCr15 steels. The effect of this tribological layer on the wear behavior of H13 steel was studied. The worn surfaces and subsurfaces of H13 steel were thoroughly characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS); the wear mechanisms were explored. The research results demonstrated that tribological layer did not exist during sliding of H13 steel with no additive, but it formed with the addition of MoS₂, Fe₂O₃ or their equiponderant mixtures. When there was no tribological layer, the wear rate rapidly increased with an increase of the load. In this case, adhesive and abrasive wear prevailed. As the additives were supplied, the artificial tribological layer was observed to be immediately formed and stably existed on worn surfaces. This tribological layer presented an obvious protective function from wear and friction. Hence, the wear rate and friction coefficient were significantly decreased. MoS₂ as tribological layer seemed to present more obvious protective function than Fe₂O₃. By supplying their mixture, the artificial tribological layer possessed not only the load-carrying capacity of Fe₂O₃, but also the lubricative capacity of MoS₂. These two simultaneous capacities could improve the friction and wear properties of H13 steel further.

Double slag process was adopted to produce low-phosphorus steel from middle-phosphorus hot metal. To achieve a stable dephosphorization operation, conventional process was modified as follows: the blowing time was extended by approximately 1 min by reducing the oxygen supply flow rate; calcium ferrite pellets were added to adjust the slag composition and viscosity; the dumping temperature was lowered by 30–50 °C by the addition of calcium ferrite pellets during the double slag process to prevent phosphorus in the slag from returning to the molten steel; and the bottom-blown gas flow was increased during the blowing process. For 40 heats of comparative experiments, the rate of dephosphorization reached 91% and ranged between 87% and 95%; the phosphorus, sulfur, manganese, and oxygen contents calculated according to the compositions of molten steel and slag as well as the temperature of molten steel at the end-point of the basic oxygen furnace process were similar to the equilibrium values for the reaction between the slag and the steel. Less free calcium oxide and metallic iron were present in the final slag, and the surface of the slag mineral phase was smooth, clear, and well developed, which showed that the slag exhibited better melting effects than that produced using the conventional slag process. A steady phosphorus capacity in the slag and stable dephosphorization effects were achieved.

To study the effect of tempering temperature on strain hardening exponent and flow stress curve, one kind of 1000 MPa grade low carbon bainitic steel for construction machinery was designed, and the standard uniaxial tensile tests were conducted at room temperature. A new flow stress model, which could predict the flow behavior of the tested steels at different tempering temperatures more efficiently, was established. The relationship between mobile dislocation density and strain hardening exponent was discussed based on the dislocation-stress relation. Arrhenius equation and an inverse proportional function were adopted to describe the mobile dislocation, and two mathematical models were established to describe the relationship between tempering temperature and strain hardening exponent. Nonlinear regression analysis was applied to the Arrhenius type model, hence, the activation energy was determined to be 37.6 kJ/mol. Moreover, the square of correlation coefficient was 0.985, which indicated a high reliability between the fitted curve and experimental data. By comparison with the Arrhenius type curve, the general trend of the inverse proportional fitting curve was coincided with the experimental data points except of some fitting errors. Thus, the Arrhenius type model can be adopted to predict the strain hardening exponent at different tempering temperatures.

The effect of dissolved niobium on the eutectoid transformation behavior in near-eutectoid high-carbon steels has been studied. Dissolved niobium is important in the eutectoid transformation behavior. It increases the eutectoid carbon content significantly (by ˜0.0477% per 0.00001% dissolved niobium), increases the hardenability of steel markedly, yields finer, more uniform, polygonal proeutectoid ferrite, induces a transition in morphology of eutectoid cementite from lamellar to somewhat spheroidal, and increases the misorientation angle of pearlite colonies from being focused near 0° to near 60°.