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

304 austenitic stainless steel was cold rolled in the range of 20%–80% reductions and then annealed at 700–900 °C for 60 s to obtain nano/ultrafine-grained (NG/UFG) structure. Transmission electron microscopy, electron backscatter diffraction and X-ray diffraction were used to characterize the resulting microstructures. The results showed that with the increase of cold reduction, the content of martensite was increased. The steel performed work hardening during cold-working owing to the occurrence of strain induced martensite which nucleated in single shear bands. Further rolling broke up the lath-type martensite into dislocation-cell type martensite because of the formation of slip bands. Samples annealed at 800–960 °C for 60 s were of NG/UFG structure with different percentage of nanocrystalline (60–100 nm) and ultrafine (100–500 nm) grains, submicron size (500–1000 nm) grains and micron size (>1000 nm) grains. The value of the Gibbs free energy exhibited that the reversion mechanism of the reversion process was shear controlled by the annealing temperature. For a certain annealing time during the reversion process, austenite nucleated first on dislocation-cell type martensite and the grains grew up subsequently and eventually to be micrometer/submicrometer grains, while the nucleation of austenite on lath-type martensite occurred later resulting in nanocrystalline/ultrafine grains. The existence of the NG/UFG structure led to a higher strength and toughness during tensile test.

The quantitative relationship between microstructure and properties of austenitic Fe-28Mn-xAl-1C (x=10 and 12 wt. %) low-density steels was evaluated using Rietveld method to refine X-ray diffraction (XRD) patterns. The results showed that a typical three-phase austenitic steel was obtained in the forged Mn28Al10 (i. e. Fe-28Mn-10Al-1C) steel, which included about 92.85 wt. % γ-Fe(Mn, Al, C) (austenite), 5.28 wt. % (Fe, Mn)₃ AlC_0.3 (κ-carbide), and 1.87 wt. % α-Fe(Al, Mn) (ferrite). For the forged Mn28Al12 (i. e. Fe-28Mn-12Al-1C) steel, nevertheless, only about 76.64 wt. % austenite, 9.63 wt. % κ-carbide, 9.14 wt. % ferrite and 4.59 wt. % Fe₃ Al (DO₃) could be obtained. Nanometer κ-carbide and DO₃ were mainly distributed in austenite grains and at the interface between austenite and ferrite, respectively. The forged Mn28Al10 steel had a better combination of strength, ductility and specific strength as compared with the forged Mn28Al12 steel. The ductility of the forged Mn28Al12 steel was far lower than that of the forged Mn28Al10 steel. The oxidation kinetics of Mn28Al10 steel oxidized at 1323 K for 5–25 h had two-stage linear rate laws, and the oxidation rate of the second stage was faster than that of the first stage. Although the oxidation kinetics of Mn28Al12 steel under this condition also had two-stage linear rate laws, the oxidation rate of the second stage was slower than that of the first stage. When the oxidation temperature increased to 1373 K, the oxidation kinetics of the two steels at 5–25 h had only one-stage linear rate law, and the oxidation rates of the two steels were far faster than those at 1323 K for 5–25 h. The oxidation resistance of Mn28Al12 steel was much better than that of Mn28Al10 steel. Ferrite layer formed between the austenite matrix and the oxidation layer of the two Fe-Mn-Al-C steels oxidized at high temperature.

As the emission control regulations get stricter, the NO_x reduction in the sintering process becomes an important environmental concern owing to its role in the formation of photochemical smog and acid rain. The NO_x emissions from the sintering machine account for 48% of total amount from the iron and steel industry. Thus, it is essential to reduce NO_x emissions from the sintering machine, for the achievement of clean production of sinter. Ca-Fe oxides, serving as the main binding phase in the sinter, are therefore used as additives into the sintering mixture to reduce NO_x emissions. The results show that the NO_x reduction ratio achieves 27.76% with 8% Ca-Fe oxides additives since the Ca-Fe oxides can advance the ignition and inhibit the nitrogen oxidation compared with the conventional condition. Meanwhile, the existence of Ca-Fe oxides was beneficial to the sinter quality since they were typical low melting point compounds. The optimal mass fraction of Ca-Fe oxides additives should be less than 8% since the permeability of sintering bed was significantly decreased with a further increase of the Ca-Fe oxides fines, inhibiting the mineralization reaction of sintering mixture. Additionally, the appropriate particle size can be obtained when mixing an equal amount of Ca-Fe oxides additives of −0. 5 mm and 0. 5−3. 0 mm in size.

The utilization of highly reactive and high-strength coke can enhance the efficiency of blast furnace by promoting indirect reduction of iron oxides. Iron compounds, as the main constituent in iron-bearing minerals, have aroused wide interest in preparation of highly reactive iron coke. However, the effects of iron compounds on pyrolysis behavior of coal and metallurgical properties of resultant cokes are still unclear. Thus, three iron compounds, i. e., Fe₃O₄, Fe₂O₃ and FeC₂O₄ · 2H₂O, were adopted to investigate their effects on coal pyrolysis behavior and metallurgical properties of the resultant cokes. The results show that iron compounds have slight effects on the thermal behavior of coal blend originated from thermogravimetric and differential thermogravimetric curves. The apparent activation energy varies with different iron compounds ranging from 94. 85 to 110. 11 kJ/mol in the primary pyrolysis process, while lower apparent activation energy is required for the secondary pyrolysis process. Iron compounds have an adverse influence on the mechanical properties and carbon structure of cokes. Strong correlations exist among coke reactivity, coke strength after reaction, and the content of metallic iron in cokes or the values of crystallite stacking height, which reflect the dependency of thermal property on metallic iron content and carbon structure of cokes.

How to manufacture the high magnetic induction grain-oriented silicon steel (Hi-B steel) by the process featured with the primary recrystallization annealing was demonstrated, during which nitriding and decarburizing were simultaneously realized in laboratory. By the techniques of optical microscope, scanning electronic microscope and electron backscattered diffraction, both the microstructure and the texture in the samples were characterized. The samples had been subjected to nitriding to different nitrogen contents at two specified temperatures using the two defined microstructural parameters : the grain size inhomogeneity factor σ* and the texture factor AR. The former is the ratio of the mean value to standard deviation of grain sizes; the latter is the ratio of the total volume fraction of the harmful textures to that of beneficial textures including {110} <001>. When the N content increased from 0. 0055 % to 0. 0330% after the annealing at both 835 and 875 °C, the resultant recrystallized grain size decreased hut σ* changed little; whilst the rise of annealing temperature from 835 to 875 °C resulted in the increase in both grain size and σ*. Moreover, either the injected N content or temperature had insignificant influence on the components of primary recrystallization texture developed during annealing. However, the increase of temperature led to the decreases in both intensity and volume fraction of {001} <120> and {110} <001> textures but increases in the {114} <481> and γ fiber textures and the resultant decrease of AR.

FeCoCrNiAl high entropy alloy coatings were prepared by supersonic air-plasma spraying. The coatings were post-treated by vacuum heat treatment at 600 and 900 °C, and laser re-melting with 300 W, respectively, to study the influence of different treatments on the structure and properties of the coatings. The phase constitution, microstructure and microhardness of the coatings after treatments were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry. Results showed that the as-sprayed coatings consisted of pure metal and Fe-Cr. The AlNi₃ phase was obtained after the vacuum heat treatment process. A body-centered cubic structure with less AlNi₃ could be found in the coating after the laser re-melting process. The average hardness values of the as-sprayed coating and the coatings with two different temperature vacuum heat treatments and with laser re-melting were 177, 227, 266 and 682 HV, respectively. This suggests that the vacuum heat treatment promoted the alloying process of the coatings, and contributed to the enhancement of the coating wear resistance. The laser re-melted coating showed the best wear resistance.

Nanocrystalline TiN films were prepared by DC reactive magnetron sputtering. The influence of substrate biases on structure, mechanical and corrosion properties of the deposited films was studied using X-ray diffraction, field emission scanning electron microscopy, nanoindentation and electrochemical techniques. The deposited films have a columnar structure, and their preferential orientation strongly depends on bias voltage. The preferential orientations change from (200) plane at low bias to (111) plane at moderate bias and then to (220) plane at relatively high bias. Nanohardness H, elastic modulus E, H/E* and H³/E*² ratios, and corrosion resistance of the deposited films increase first and then decrease with the increase in bias voltage. All the best values appear at bias of −120 V, attributing to the film with a fine, compact and less defective structure. This demonstrates that there is a close relation among microstructure, mechanical and corrosion properties of the TiN films, and the film with the best mechanical property can also provide the most effective corrosion protection.

A model of deformation resistance during hot strip rolling was established based on generalized additive model. Firstly, a data modeling method based on generalized additive model was given. It included the selection of dependent variable and independent variables of the model, the link function of dependent variable and smoothing functional form of each independent variable, estimating process of the link function and smooth functions, and the last model modification. Then, the practical modeling test was carried out based on a large amount of hot rolling process data. An integrated variable was proposed to reflect the effects of different chemical compositions such as carbon, silicon, manganese, nickel, chromium, niobium, etc. The integrated chemical composition, strain, strain rate and rolling temperature were selected as independent variables and the cubic spline as the smooth function for them. The modeling process of deformation resistance was realized by SAS software, and the influence curves of the independent variables on deformation resistance were obtained by local scoring algorithm. Some interesting phenomena were found, for example, there is a critical value of strain rate, and the deformation resistance increases before this value and then decreases. The results confirm that the new model has higher prediction accuracy than traditional ones and is suitable for carbon steel, microalloyed steel, alloyed steel and other steel grades.

The microstructure and mechanical properties of dissimilar joints of AISI 316L austenitic stainless steel and API X70 high-strength low-alloy steel were investigated. For this purpose, gas tungsten arc welding (GTAW) was used in three different heat inputs, including 0.73, 0. 84, and 0. 97 kJ/mm. The microstructural investigations of different zones including base metals, weld metal, heat-affected zones and interfaces were performed by optical microscopy and scanning electron microscopy. The mechanical properties were measured by microhardness, tensile and impact tests. It was found that with increasing heat input, the dendrite size and inter-dendritic spacing in the weld metal increased. Also, the amount of delta ferrite in the weld metal was reduced. Therefore, tensile strength and hardness were reduced and impact test energy was increased. The investigation of the interface between AISI 316L base metal and ER316L filler metal showed that increasing the heat input increases the size of austenite grains in the fusion boundary. A transition region was formed at the interface between API X70 steel and filler metals.

Materials data deep-excavation is very important in materials genome exploration. In order to carry out materials data deep-excavation in hot die steels and obtain the relationships among alloying elements, heat treatment parameters and materials properties, a 11 × 12 × 12 × 4 back-propagation (BP) artificial neural network (ANN) was set up. Alloying element contents, quenching and tempering temperatures were selected as input; hardness, tensile and yield strength were set as output parameters. The ANN shows a high fitting precision. The effects of alloying elements and heat treatment parameters on the properties of hot die steel were studied using this model. The results indicate that high temperature hardness increases with increasing alloying element content of C, Si, Mo, W, Ni, V and Cr to a maximum value and decreases with further increase in alloying element content. The ANN also predicts that the high temperature hardness will decrease with increasing quenching temperature, and possess an optimal value with increasing tempering temperature. This model provides a new tool for novel hot die steel design.