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

The effects of deforming temperatures on the tensile behaviors of quenching and partitioning treated steels were investigated. It was found that the ultimate tensile strength of the steel decreased with the increasing temperature from 25 to 100 °C, reached the maximum value at 300 °C, and then declined by a significant extent when the temperature further reached 400 °C. The total elongations at 100, 200 and 300 °C are at about the same level. The steel achieved optimal mechanical properties at 300 °C due to the proper transformation behavior of retained austenite since the stability of retained austenite is largely dependent on the deforming temperature. When tested at 100 and 200 °C, the retained austenite was reluctant to transform, while at the other temperatures, about 10 vol. % of retained austenite transformed during the tensile tests. The relationship between the stability of retained austenite and the work hardening behavior of quenching and partitioning treated steels at different deforming temperatures was also studied and discussed in detail. In order to obtain excellent mechanical properties, the stability of retained austenite should be carefully controlled so that the effect of transformation-induced plasticity could take place continuously during plastic deformation.

Coherent jets are widely used in electric arc furnace (EAF) steelmaking to increase the oxygen utilization and chemical reaction rates. However, the influence of fuel gas combustion on jet behavior is not fully understood yet. The flow and combustion characteristics of a coherent jet were thus investigated at steelmaking temperature using Fluent software, and a detailed chemical kinetic reaction mechanism was used in the combustion reaction model. The axial velocity and total temperature of the supersonic jet were measured via hot state experiments. The simulation results were compared with the experimental data and the empirical jet model proposed by Ito and Muchi and good consistency was obtained. The research results indicated that the potential core length of the coherent jet can be prolonged by optimizing the combustion effect of the fuel gas. Besides, the behavior of the supersonic jet in the subsonic section was also investigated, as it is an important factor for controlling the position of the oxygen lance. The investigation indicated that the attenuation of the coherent jet is more notable than that of the conventional jet in the subsonic section.

A statistic method, statistics of extreme values (SEV), was described in detail, which can estimate the size of maximum inclusion in steel. The characteristic size of the maximum inclusion in a high clean bearing steel (GCr15) was evaluated by this method, and the morphology and composition of large inclusions found were analyzed by scanning electron microscopy (SEM). When standard inspection area (S₀) is 280 mm² the characteristic size of the biggest inclusion found in 30 standard inspection area is 23.93 µm, and it has a 99.9% probability of the characteristic size of maximum inclusion predicted being no larger than 36.85 pm in the experimental steel. SEM result shows that large inclusions found are mainly composed of CaS, calcium-aluminate and MgO. Compositing widely exists in large inclusions in high clean bearing steel. Compared with traditional evaluation method, SEV method mainly focuses on inclusion size, and the estimation result is not affected by inclusion types. SEV method is suitable for the inclusion evaluation of high clean bearing steel.

The microstructural evolutions of 5Mn steel during various heat treatments have been investigated by in-situ transmission electron microscopy (TEM). The specimen of 5Mn steel was prepared using focused ion beam (FIB) milling, which allowed the selection of specific morphology of interest prior to the in-situ observation. The complete austenization at 800 °C was verified at the atomic scale by minimizing thermal expansion and sample drift in a heating holder based on micro-electro-mechanical-systems. During annealing at 650 °C, the formation of reverted austenite was dynamically observed, while the morphologies of austenite laths of 5Mn steel after in-situ heating were quite similar to that after ex-situ intercritical annealing. During annealing at 500 °C, the morphological evolution of cementite and associated Mn diffusion were investigated. It was demonstrated that a combination of FIB sampling and high temperature in-situ TEM enables us to probe the morphological evolution and elemental diffusion of specific areas of interest in steel at high spatial resolution.

The Lüders deformation behavior in a medium Mn transformation-induced plasticity (TRIP) steel is investigated at different temperatures ranging from 25 to 300 °C. It demonstrates that the Lüders band appears at all testing temperatures but with varied Lüders strains which do not change monotonically with temperature. The martensitic transformation is simultaneously observed within the Lüders band in varying degrees depending on the testing temperature. It is well verified that the martensitic transformation is not responsible for the formation of Lüders band, and a reasonable explanation is given for the non-monotonic variation of Lüders strain with increasing temperature.

Abstract

A technique comprising coal-based direct reduction followed by magnetic separation was presented to recover iron and copper from copper slag flotation tailings. Optimal process parameters, such as reductant and additive ratios, reduction temperature, and reduction time, were experimentally determined and found to be as follows: a limestone ratio of 25%, a bitumite ratio of 30%, and reduction roasting at 1473 K for 90 min. Under these conditions, copper-bearing iron powders (CIP) with an iron content of 90.11% and copper content of 0.86%, indicating iron and copper recoveries of 87.25% and 83.44% respectively, were effectively obtained. Scanning electron microscopy and energy dispersive spectroscopy of the CIP revealed that some tiny copper particles were embedded in metal iron and some copper formed alloy with iron, which was difficult to achieve the separation of these two metals. Thus, the copper went into magnetic products by magnetic separation. Adding copper into the steel can produce weathering steel. Therefore, the CIP can be used as an inexpensive raw material for weathering steel.

Abstract

Al-Si coated ultra-high strength steel (UHSS) has been commonly applied in hot stamping process. The influence of austenitizing temperature on microstructure of Al-Si coating of UHSS during hot stamping process and its tribological behavior against H13 steel under elevated temperature were simulatively investigated. The austenitizing temperature of Al-Si coated UHSS and its microstructual evolution were confirmed and analyzed by differential scanning calorimetry and scanning electron microscopy. A novel approach to tribological testing by replicating hot stamping process temperature history was presented. Results show that the hard and stable phases Fe₂Al₅ + FeAl₂ formed on Al-Si coating surface after exposure to 930 °C for 5 min, which was found to be correlated to the tribological behavior of coating. The friction coefficient of coated steel was more stable and higher than that of uncoated one. The main wear mechanism of Al-Si coated UHSS was adhesion wear, while abrasive wear was dominant for the uncoated UHSS.

Abstract

Modification of metallurgical coke, biomass char and semi-coke was carried out using a microwave device with power of 450–850 W and irradiation time of 6–12 min. The desulfurization rates of three carbon materials before and after modification were tested. The effects of microwave power and irradiation time on the pore texture and surface chemical characteristics of the three carbon materials were examined by SEM, BET and Fourier transform infrared spectroscopy (FTIR). The results showed that the specific surface area, total pore volume and pore diameter of biomass char and semi-coke after irradiation decreased slightly. Noteworthily, the pore diameter turned small and the acidic functional groups on their surface decomposed, thereby the basicity of carbon surface increased by microwave modification. The optimal promotion of desulfurization rate of three carbon materials was semi-coke irradiated at 850 W for 9 min and the sulfur dioxide adsorption rate was up to 45%.

Abstract

A new analysis method based on serial sectioning and three-dimensional (3D) reconstruction was developed to characterize the mineral microstructure of iron ore sinter. Through the 3D reconstruction of two types of iron ore sinters, the morphology and distribution of minerals in three-dimensional space were analyzed, and the volume fraction of minerals in a 3D image was calculated based on their pixel points. In addition, the microhardness of minerals was measured with a Vickers hardness tester. Notably, different mineral compositions and distributions are obtained in these two sinters. The calcium ferrite in Sinter 1 is dendritic with many interconnected pores, and these grains are crisscrossed and interwoven; the calcium ferrite in Sinter 2 is strip shaped and interweaves with magnetite, silicate and columnar pores. The calculated mineral contents based on a two-dimensional region are clearly different among various layers. Quantitative analysis shows that Sinter 1 contains a greater amount of calcium ferrite and hematite, whereas Sinter 2 contains more magnetite and silicate. The microhardness of minerals from highest to lowest is hematite, calcium ferrite, magnetite and silicate. Thus, Sinter 1 has a greater tumbler strength than Sinter 2.

Abstract

Through molecular dynamics simulations, the mechanical behavior of nanoporous copper under impact loading was investigated with relative densities ranging from 77.91% to 98.36%, focusing on deformation mechanism, the scaling laws and influence of ligament sizes. Results show that the classical Gibson-Ashby's scaling laws should be modified for prediction of both the Young's modulus and yield stress. A proportional relationship is established between cell wall thickness and yield stress, and new modified scaling equations are built for nanoporous copper with consideration on both relative mass density and size effects of ligaments. The size effect can be explained by larger surface area/volume ratio of samples with thinner ligament size and limited dislocation source activation due to narrow space between larger numbers of voids.